So my party are about to fight a demilich, they know they're about to fight a demilich and are intent on fighting it. Now the issue is, my party's strategy in combat has generally been: fireball, smites and standing close to the Paladin for the bonus to saving throws. Whilst normally quite effective, the traits of a demilich means they might want to adjust their strategy this time around. So I thought I'd have an NPC, with knowledge of what a demilich is, tell them its abilities and traits.
Now some of the traits are easy to explain: "It is small and nimble", "Your attempts to turn it like a common zombie will fail", "If you stand too close to it, it can sustain itself by draining your life energy".
The problem comes from Avoidance and Legendary Resistances. For those who don't have access to the Monster Manual, Avoidance is a trait similar to the Evasion ability Rogues and Monks get. When a creature with this trait makes a saving throw to take only half damage against an effect, it takes half the usual damage on a failure and no damage on a success, this trait applies to all saving throws, not just dexterity saving throws.
Now the issue is with how to explain this trait in-character. Obviously I can't have an NPC say terms like 'Saving Throws'. The same applies for describing Legendary Resistance. So fellow DM's, how would you go about this issue?
P.S. Looking at this statblock, I'm surprised Demilich's don't have any languages or even telepathy yet have an Intelligence of 20. It doesn't even have the "Understands language but cannot speak" trait. I'm fairly certain in Neverwinter Nights: Hordes of the Underdark you do engage in conversation with one, though this might be an edition change.
Unpopular opinion; you are being way too kind to your players by giving them an NPC that knows about the abilities of a demilich. I'd give the characters vague information that previous attempts to defeat the demilich have failed. Seasoned adventuring groups that were known to have solid tactics were soundly defeated. Hopefully, this triggers to the players that they need to research before combat.
Now, however they get such information, why would they have full knowledge of every ability? That is kind of the cheat sheet of what the monster can and cannot do. A sage might be able to provide the basics. But the PC's need to prepare that they may need to adjust in combat to abilities they didn't account for.
When a group is powerful enough to take on a demilich, time to take the training wheels off. They should (rightfully so) realize they need to research before rushing in to combat.
I'm surprised they haven't already discovered the limitations of those tactics against something like a dragon (not being in the area is better than having a bonus to saves).
"Small and nimble" already seems to cover Avoidance to me, and Legendary Resistances just means it's succeeding on more saving throws and requires no explanation ahead of time
You really shouldn't just spell out every ability a monster has for your players -- you are denying them the fun/challenge of having to adjust their tactics on the fly when they discover Plan A doesn't work
If you are intent on introducing this NPC, have them be the only survivor of a previous encounter with a demilich who can tell them what doesn't work, rather than what does. "Yeah, we kept a tight group around our paladin too, to benefit from her aura. Poor Eleanor, she went down first, trying to buy the rest of us time. I only escaped by being at the back"
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Active characters:
Carric Aquissar, elven wannabe artist in his deconstructionist period (Archfey warlock) Lan Kidogo, mapach archaeologist and treasure hunter (Knowledge cleric) Mardan Ferres, elven private investigator obsessed with that one unsolved murder (Assassin rogue) Xhekhetiel, halfling survivor of a Betrayer Gods cult (Runechild sorcerer/fighter)
I see a lot of people saying I'm being too kind by giving them this information, but this group I'm dming is this groups first campaign ever. The group consists of 3 level 11's so going in unprepared against a CR 18 enemy is going to be an incredibly difficult challenge. Considering one of the abilities of a demilich is "All creatures within 10ft make a DC 15 con save or lose all hp" and they tend to all stay within 10ft of each other, I feel it's only fair they're able to prepare themselves as much as possible.
Can I ask: why do you want to give away this information? Demiliches are scary enemies, and introducing the players to their unique abilities is part of the fun. Legendary resistance becomes less interesting and more of a metagamey issue when players see it coming in advance. Giving away the demilich's attacks is more defensible, in my opinion.
Can I ask: why do you want to give away this information? Demiliches are scary enemies, and introducing the players to their unique abilities is part of the fun. Legendary resistance becomes less interesting and more of a metagamey issue when players see it coming in advance. Giving away the demilich's attacks is more defensible, in my opinion.
Because I'm fairly certain if I don't it'll be a TPK, whereas if I give this party the information in advance there's only a chance of a TPK. If this was a lower CR enemy then wasting a turn or two realising a method of attack doesn't work wouldn't be a problem. It's not really the legendary resistance I want to give away, it's the legendary resistance combined with avoidance that means the majority of the cleric spells prepared won't deal any damage at all, at least 3 times.
If this was my other group, then I'd have no problem, they've shown they can adapt, but this group still haven't grasped the mechanics that well. Some might say I need to not baby them, but the party aren't exactly... optimised, and a NPC telling them "If an enemy can fly you may want to use a flying spell to hit with your smites", "While you normally stay together in combat, you might want to think again for this foe" and "Your spells that always do some damage will likely be useless at first" may make them think they need to change a few things.
Ultimately I believe it comes down to that I believe there has to be a certain minimum chance of victory in a fight. I strongly believe, based on my experience dm'ing this particular group for over a year, that they will not pass that minimum chance without at least knowing some of the traits. This will also be their first opportunity to actually prepare before fighting a boss, because the rest they've either been ambushed or stumbled into whilst exploring a dungeon.
Can I ask: why do you want to give away this information? Demiliches are scary enemies, and introducing the players to their unique abilities is part of the fun. Legendary resistance becomes less interesting and more of a metagamey issue when players see it coming in advance. Giving away the demilich's attacks is more defensible, in my opinion.
Because I'm fairly certain if I don't it'll be a TPK, whereas if I give this party the information in advance there's only a chance of a TPK. If this was a lower CR enemy then wasting a turn or two realising a method of attack doesn't work wouldn't be a problem. It's not really the legendary resistance I want to give away, it's the legendary resistance combined with avoidance that means the majority of the cleric spells prepared won't deal any damage at all, at least 3 times.
If this was my other group, then I'd have no problem, they've shown they can adapt, but this group still haven't grasped the mechanics that well. Some might say I need to not baby them, but the party aren't exactly... optimised, and a NPC telling them "If an enemy can fly you may want to use a flying spell to hit with your smites", "While you normally stay together in combat, you might want to think again for this foe" and "Your spells that always do some damage will likely be useless at first" may make them think they need to change a few things.
Ultimately I believe it comes down to that I believe there has to be a certain minimum chance of victory in a fight. I strongly believe, based on my experience dm'ing this particular group for over a year, that they will not pass that minimum chance without at least knowing some of the traits. This will also be their first opportunity to actually prepare before fighting a boss, because the rest they've either been ambushed or stumbled into whilst exploring a dungeon.
You know your table, so any input from others can only speak to the general. But a group that hasn't grasped mechanics that well probably should not be fighting a complex encounter like a demilich. If you have an NPC alert them to what resistances ad abilities they need to prepare for, without them doing any research on their own; is it really a victory? What I'd be careful about is giving away free information like BBEG abilities without such information being earned. Should they ever get surprised in an encounter later on, it may cause frustration because you didn't tell them prior the monster had an ability they were not aware of.
Instead of an NPC, try letting the PCs into a library beforehand so they can research the demilich's powers. They might find the writing of someone who studies undead, an adventurer who fought one of these creatures, or even a lich who researched demiliches to prepare to potentially become one. It could say something like this (spoiler blind put in to prevent the post from being too long):
"The ancient skull of the Dark Mage Clautherius descended upon us, howling menacingly. Our cleric, speaking a prayer to her patron god, threw a beam of divine light at the undead, but to no avail. The wizard, the fighter, and myself were next to act, they shooting it with arrows and fireballs, and I with crackling magical beams drawn from the pits of hell. But the demilich, with dexterity beyond the capability of all but the most skilled humans, evaded all but the best placed of our attacks. While the four of us failingly attempted to pin the creature down, it flew among us, sending dark tendrils of necrotic energy from its eye sockets and draining from us the life force that it used to repair what little damage we managed to deal to its magically empowered shell. As we weakened, it let out another scream, and the cleric dropped to the floor, briefly bloodied and screaming and then dead. I, alone, was able to make it out that day. Whether I will exact my revenge on the behalf of my patron Bel and my fallen allies is undetermined, but I know that I will stop at nothing to try."
Translation: A cleric, wizard, warlock, and fighter go into a demilich's lair and attack it. It uses a combination of Howl, Life Drain, and its defensive features to beat the crap out of them and kill all of them except the warlock, who barely escapes with his life. For one thing, it's pretty interesting stuff that makes the players feel immersed in the campaign world, and for another, it offers hints at the demilich's abilities (it is elusive, hard to turn, can deal necrotic damage to heal itself, et cetra) without revealing everything. After reading a few similar texts from different sources about demiliches, the party will feel ready to fight it.
P. S. I decided that demiliches not being able to speak is dumb, and let them speak the languages that they knew in life. They have 20 INT, so they should remember how to speak.
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Panda-wat (I hate my username) is somehow convinced that he is objectively right about everything D&D related even though he obviously is not. Considering that, he'd probably make a great D&D youtuber.
"If I die, I can live with that." ~Luke Hart, the DM lair
Unpopular opinion; you are being way too kind to your players by giving them an NPC that knows about the abilities of a demilich. I'd give the characters vague information that previous attempts to defeat the demilich have failed. Seasoned adventuring groups that were known to have solid tactics were soundly defeated. Hopefully, this triggers to the players that they need to research before combat.
Now, however they get such information, why would they have full knowledge of every ability? That is kind of the cheat sheet of what the monster can and cannot do. A sage might be able to provide the basics. But the PC's need to prepare that they may need to adjust in combat to abilities they didn't account for.
When a group is powerful enough to take on a demilich, time to take the training wheels off. They should (rightfully so) realize they need to research before rushing in to combat.
I entirely agree cr does assume some fumbling around with resistances and immunities so you'll likely find combats are allot easier than expected.
There are features, spells and ability checks which can be made to give this kind of information. For particularly powerful monsters like a demi lich there should be hints to their identity around the environment to let players make these kinds of checks and things before combat.
Insight the would be the skill I would use in combat to get information on how a monster fights so they can always make an insight check and such checks don't really require an action in combat but can be tied to other actions. For example when you make an attack you could make an insight check as part of it to see how they respond. Basically there is no check action but they need to explain what they are doing to try and tease out the information whether its taking no action to watch them intensely or trying to bait it out with an attack.
I'm a bit late to this but to expand on other ideas previously given a bit.
Whilst working their through a dungeon with a Demilich at the end and assuming you are using the Lair Traits/Actions:
Describe the ever growing sense of foreboding the area gives off. If anyone has a high passive Insight score the describe them feeling almost palpable waves of Hate, Anger and Despair washing over them as they progress. If no-one has a high passive Inisght then allow anyone proficient with it to do a Insight check with a DC15 for success to attune to the emotions in the area.
Anyone with some connection to banishing undead (Clerics and Paladins for the most part but includes any spell caster with access to Undead summoning) can ascvertain that the negative emotions that permeate the area will likely bolster undead, you can use either passive Arcana to rely this or ask them to make Arcana checks. This will give them information about one of the traits associated with a Demiliches Lair.
Let them find the remains of previous adventurers, if the party want to they can use Speak With Dead to find out information, maybe pick up a journal with some entries detailing their research or encounters within the dungeon.
As the party enter it for the first time they feel the air sucked out of their body and as it leaves they feel themesleves diminished body and soul (to represent the 3d10 necrotic damage they take on entering).
Have arcane sigils burned, scarred and carved into various surfaces, Arcana and Religion checks can reveal they are wards to prevent magical transportation into or out of the area. The degree of success could reveal effects such as Misty Step upto Plane Shift would almost useless in the area, if they succeed really well then give them the info that the sigils prevent travel out of the dungeon but they could still teleport within it. This should set up some alarm bells as the party now know they have no way to exit quickly.
At or near the entrance to the dungeon have a ward that duplicates the Alarm spell. This was one of the outer defences the Lich set up before devolving into a Demilich, once it is set off the Demilich is roused and aware of intruders. Additionally door and secret chambers locked potentailly ceturies ago open allowing previously created zombies and skeletons to move into the area to search and destroy intruders, to really hammer home how powerful the Demilich used to be have a Zombie Beholder floating around and to give the Party a moment of "well that was easy" have them fight a floating skull using the Flameskull but its not on fire and I might also be tempted to have a Banshee knocking around as well.
For descriptive purposes when the Demilich uses its Howl ability you can describe a sudden amplification of all the negative emotions they have felt in the dungeon and the sudden onrush of hopelessness that causes their hearts to beating stop if only for a brief moment. For the Life Drain have tendrils of black and grey vapour pour out of them and flow into the Demilich, describing their body and soul diminishing much like it did when they first entered the dungeon.
Should they defeat the Demilich have the skull explode into dust, ash and smoke.....and then see if tehy decide to try and lcoate its Phylactary, if they don't, ask one of the party to roll a d10 and don't tell them why.
Because I'm fairly certain if I don't it'll be a TPK, whereas if I give this party the information in advance there's only a chance of a TPK. If this was a lower CR enemy then wasting a turn or two realising a method of attack doesn't work wouldn't be a problem. It's not really the legendary resistance I want to give away, it's the legendary resistance combined with avoidance that means the majority of the cleric spells prepared won't deal any damage at all, at least 3 times.
If this was my other group, then I'd have no problem, they've shown they can adapt, but this group still haven't grasped the mechanics that well. Some might say I need to not baby them, but the party aren't exactly... optimised, and a NPC telling them "If an enemy can fly you may want to use a flying spell to hit with your smites", "While you normally stay together in combat, you might want to think again for this foe" and "Your spells that always do some damage will likely be useless at first" may make them think they need to change a few things.
Ultimately I believe it comes down to that I believe there has to be a certain minimum chance of victory in a fight. I strongly believe, based on my experience dm'ing this particular group for over a year, that they will not pass that minimum chance without at least knowing some of the traits. This will also be their first opportunity to actually prepare before fighting a boss, because the rest they've either been ambushed or stumbled into whilst exploring a dungeon.
That's a fair reason. One idea that comes to mind is to reflavor legendary resistance as being some sort of magical defense system. Reveal to the party that in its lair, the demilich has three runes that provide strong magical resistance to a wide variety of effects. For avoidance, flavor it as the demilich being partially in spirit form, and thus either suffers less from area effects or phases out of existence momentarily to entirely avoid attacks. The key is that flavoring things properly will make them easier to swallow.
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So my party are about to fight a demilich, they know they're about to fight a demilich and are intent on fighting it. Now the issue is, my party's strategy in combat has generally been: fireball, smites and standing close to the Paladin for the bonus to saving throws. Whilst normally quite effective, the traits of a demilich means they might want to adjust their strategy this time around. So I thought I'd have an NPC, with knowledge of what a demilich is, tell them its abilities and traits.
Now some of the traits are easy to explain: "It is small and nimble", "Your attempts to turn it like a common zombie will fail", "If you stand too close to it, it can sustain itself by draining your life energy".
The problem comes from Avoidance and Legendary Resistances. For those who don't have access to the Monster Manual, Avoidance is a trait similar to the Evasion ability Rogues and Monks get. When a creature with this trait makes a saving throw to take only half damage against an effect, it takes half the usual damage on a failure and no damage on a success, this trait applies to all saving throws, not just dexterity saving throws.
Now the issue is with how to explain this trait in-character. Obviously I can't have an NPC say terms like 'Saving Throws'. The same applies for describing Legendary Resistance. So fellow DM's, how would you go about this issue?
P.S. Looking at this statblock, I'm surprised Demilich's don't have any languages or even telepathy yet have an Intelligence of 20. It doesn't even have the "Understands language but cannot speak" trait. I'm fairly certain in Neverwinter Nights: Hordes of the Underdark you do engage in conversation with one, though this might be an edition change.
Unpopular opinion; you are being way too kind to your players by giving them an NPC that knows about the abilities of a demilich. I'd give the characters vague information that previous attempts to defeat the demilich have failed. Seasoned adventuring groups that were known to have solid tactics were soundly defeated. Hopefully, this triggers to the players that they need to research before combat.
Now, however they get such information, why would they have full knowledge of every ability? That is kind of the cheat sheet of what the monster can and cannot do. A sage might be able to provide the basics. But the PC's need to prepare that they may need to adjust in combat to abilities they didn't account for.
When a group is powerful enough to take on a demilich, time to take the training wheels off. They should (rightfully so) realize they need to research before rushing in to combat.
I'm surprised they haven't already discovered the limitations of those tactics against something like a dragon (not being in the area is better than having a bonus to saves).
Avoidance:
At best, spells and similar area effects that sometimes do much and other times do little, will either do little or nothing.
--------
Legendary Resistances:
As you know, creatures of such legendary powers will always take the lesser of several possible effects until you exhaust it's power.
"Small and nimble" already seems to cover Avoidance to me, and Legendary Resistances just means it's succeeding on more saving throws and requires no explanation ahead of time
You really shouldn't just spell out every ability a monster has for your players -- you are denying them the fun/challenge of having to adjust their tactics on the fly when they discover Plan A doesn't work
If you are intent on introducing this NPC, have them be the only survivor of a previous encounter with a demilich who can tell them what doesn't work, rather than what does. "Yeah, we kept a tight group around our paladin too, to benefit from her aura. Poor Eleanor, she went down first, trying to buy the rest of us time. I only escaped by being at the back"
Active characters:
Carric Aquissar, elven wannabe artist in his deconstructionist period (Archfey warlock)
Lan Kidogo, mapach archaeologist and treasure hunter (Knowledge cleric)
Mardan Ferres, elven private investigator obsessed with that one unsolved murder (Assassin rogue)
Xhekhetiel, halfling survivor of a Betrayer Gods cult (Runechild sorcerer/fighter)
I see a lot of people saying I'm being too kind by giving them this information, but this group I'm dming is this groups first campaign ever. The group consists of 3 level 11's so going in unprepared against a CR 18 enemy is going to be an incredibly difficult challenge. Considering one of the abilities of a demilich is "All creatures within 10ft make a DC 15 con save or lose all hp" and they tend to all stay within 10ft of each other, I feel it's only fair they're able to prepare themselves as much as possible.
Thanks a bunch, your answers were helpful.
Can I ask: why do you want to give away this information? Demiliches are scary enemies, and introducing the players to their unique abilities is part of the fun. Legendary resistance becomes less interesting and more of a metagamey issue when players see it coming in advance. Giving away the demilich's attacks is more defensible, in my opinion.
Because I'm fairly certain if I don't it'll be a TPK, whereas if I give this party the information in advance there's only a chance of a TPK. If this was a lower CR enemy then wasting a turn or two realising a method of attack doesn't work wouldn't be a problem. It's not really the legendary resistance I want to give away, it's the legendary resistance combined with avoidance that means the majority of the cleric spells prepared won't deal any damage at all, at least 3 times.
If this was my other group, then I'd have no problem, they've shown they can adapt, but this group still haven't grasped the mechanics that well. Some might say I need to not baby them, but the party aren't exactly... optimised, and a NPC telling them "If an enemy can fly you may want to use a flying spell to hit with your smites", "While you normally stay together in combat, you might want to think again for this foe" and "Your spells that always do some damage will likely be useless at first" may make them think they need to change a few things.
Ultimately I believe it comes down to that I believe there has to be a certain minimum chance of victory in a fight. I strongly believe, based on my experience dm'ing this particular group for over a year, that they will not pass that minimum chance without at least knowing some of the traits. This will also be their first opportunity to actually prepare before fighting a boss, because the rest they've either been ambushed or stumbled into whilst exploring a dungeon.
You know your table, so any input from others can only speak to the general. But a group that hasn't grasped mechanics that well probably should not be fighting a complex encounter like a demilich. If you have an NPC alert them to what resistances ad abilities they need to prepare for, without them doing any research on their own; is it really a victory? What I'd be careful about is giving away free information like BBEG abilities without such information being earned. Should they ever get surprised in an encounter later on, it may cause frustration because you didn't tell them prior the monster had an ability they were not aware of.
Instead of an NPC, try letting the PCs into a library beforehand so they can research the demilich's powers. They might find the writing of someone who studies undead, an adventurer who fought one of these creatures, or even a lich who researched demiliches to prepare to potentially become one. It could say something like this (spoiler blind put in to prevent the post from being too long):
"The ancient skull of the Dark Mage Clautherius descended upon us, howling menacingly. Our cleric, speaking a prayer to her patron god, threw a beam of divine light at the undead, but to no avail. The wizard, the fighter, and myself were next to act, they shooting it with arrows and fireballs, and I with crackling magical beams drawn from the pits of hell. But the demilich, with dexterity beyond the capability of all but the most skilled humans, evaded all but the best placed of our attacks. While the four of us failingly attempted to pin the creature down, it flew among us, sending dark tendrils of necrotic energy from its eye sockets and draining from us the life force that it used to repair what little damage we managed to deal to its magically empowered shell. As we weakened, it let out another scream, and the cleric dropped to the floor, briefly bloodied and screaming and then dead. I, alone, was able to make it out that day. Whether I will exact my revenge on the behalf of my patron Bel and my fallen allies is undetermined, but I know that I will stop at nothing to try."
Translation: A cleric, wizard, warlock, and fighter go into a demilich's lair and attack it. It uses a combination of Howl, Life Drain, and its defensive features to beat the crap out of them and kill all of them except the warlock, who barely escapes with his life. For one thing, it's pretty interesting stuff that makes the players feel immersed in the campaign world, and for another, it offers hints at the demilich's abilities (it is elusive, hard to turn, can deal necrotic damage to heal itself, et cetra) without revealing everything. After reading a few similar texts from different sources about demiliches, the party will feel ready to fight it.
P. S. I decided that demiliches not being able to speak is dumb, and let them speak the languages that they knew in life. They have 20 INT, so they should remember how to speak.
Panda-wat (I hate my username) is somehow convinced that he is objectively right about everything D&D related even though he obviously is not. Considering that, he'd probably make a great D&D youtuber.
"If I die, I can live with that." ~Luke Hart, the DM lair
I entirely agree cr does assume some fumbling around with resistances and immunities so you'll likely find combats are allot easier than expected.
There are features, spells and ability checks which can be made to give this kind of information. For particularly powerful monsters like a demi lich there should be hints to their identity around the environment to let players make these kinds of checks and things before combat.
Insight the would be the skill I would use in combat to get information on how a monster fights so they can always make an insight check and such checks don't really require an action in combat but can be tied to other actions. For example when you make an attack you could make an insight check as part of it to see how they respond. Basically there is no check action but they need to explain what they are doing to try and tease out the information whether its taking no action to watch them intensely or trying to bait it out with an attack.
I'm a bit late to this but to expand on other ideas previously given a bit.
Whilst working their through a dungeon with a Demilich at the end and assuming you are using the Lair Traits/Actions:
Describe the ever growing sense of foreboding the area gives off. If anyone has a high passive Insight score the describe them feeling almost palpable waves of Hate, Anger and Despair washing over them as they progress. If no-one has a high passive Inisght then allow anyone proficient with it to do a Insight check with a DC15 for success to attune to the emotions in the area.
Anyone with some connection to banishing undead (Clerics and Paladins for the most part but includes any spell caster with access to Undead summoning) can ascvertain that the negative emotions that permeate the area will likely bolster undead, you can use either passive Arcana to rely this or ask them to make Arcana checks. This will give them information about one of the traits associated with a Demiliches Lair.
Let them find the remains of previous adventurers, if the party want to they can use Speak With Dead to find out information, maybe pick up a journal with some entries detailing their research or encounters within the dungeon.
As the party enter it for the first time they feel the air sucked out of their body and as it leaves they feel themesleves diminished body and soul (to represent the 3d10 necrotic damage they take on entering).
Have arcane sigils burned, scarred and carved into various surfaces, Arcana and Religion checks can reveal they are wards to prevent magical transportation into or out of the area. The degree of success could reveal effects such as Misty Step upto Plane Shift would almost useless in the area, if they succeed really well then give them the info that the sigils prevent travel out of the dungeon but they could still teleport within it. This should set up some alarm bells as the party now know they have no way to exit quickly.
At or near the entrance to the dungeon have a ward that duplicates the Alarm spell. This was one of the outer defences the Lich set up before devolving into a Demilich, once it is set off the Demilich is roused and aware of intruders. Additionally door and secret chambers locked potentailly ceturies ago open allowing previously created zombies and skeletons to move into the area to search and destroy intruders, to really hammer home how powerful the Demilich used to be have a Zombie Beholder floating around and to give the Party a moment of "well that was easy" have them fight a floating skull using the Flameskull but its not on fire and I might also be tempted to have a Banshee knocking around as well.
For combat with the Demilich itself its worth reading the monsters know what they are doing, link here: https://www.themonstersknow.com/undead-tactics-demiliches/
For descriptive purposes when the Demilich uses its Howl ability you can describe a sudden amplification of all the negative emotions they have felt in the dungeon and the sudden onrush of hopelessness that causes their hearts to beating stop if only for a brief moment. For the Life Drain have tendrils of black and grey vapour pour out of them and flow into the Demilich, describing their body and soul diminishing much like it did when they first entered the dungeon.
Should they defeat the Demilich have the skull explode into dust, ash and smoke.....and then see if tehy decide to try and lcoate its Phylactary, if they don't, ask one of the party to roll a d10 and don't tell them why.
That's a fair reason. One idea that comes to mind is to reflavor legendary resistance as being some sort of magical defense system. Reveal to the party that in its lair, the demilich has three runes that provide strong magical resistance to a wide variety of effects. For avoidance, flavor it as the demilich being partially in spirit form, and thus either suffers less from area effects or phases out of existence momentarily to entirely avoid attacks. The key is that flavoring things properly will make them easier to swallow.