PLOT TWIST: The NPC that was helping them the whole way, IS the Phylactery!
This actually technically is not impossible, a phylactery can theoretically be anything...

Quote from Cleseus_Of_Castigonia >>

PLOT TWIST: The NPC that was helping them the whole way, IS the Phylactery!
This actually technically is not impossible, a phylactery can theoretically be anything...

 It happens in one of the adventures of anthology adventure books. 

I like using a ring of shielding. An identity will show a ring of shielding with a soul in it. Divination spells will fail to tell what it really is (except with true sight)  then give it as a magic items to the party mixed with others. 

I love the idea of a Lich using a large, important structure as their Phylactery, preferably one the party likes, or which is impossible to destroy without causing huge cultural harm. A dam holding back a huge lake. The university library, containing all the books ever put to print. The forest itself. A Mountain. The Basilica of St Peter. The Gaudi Cathedral. Notre Dame. The Empire State Building.

Something that is fundamentally simple to plan to destroy (IE doesn't require magical muckery), but which can change the entire world if it is.

this probably wouldn't work rules as written, but it was for a one shot so i made it that the warforged that they encountered at the beginning of the one shot WAS the phylactery and didn't know it, hence why the lich was desperate to find him after the warforged went missing before anyone figured out.

It could be a part of the liches body itself, such as its skull. It ensures that if the players want to defeat the lich, they have to fight em. On the other hand, making the liches only weakness a part of its body would be a stupid move on his part. 

Vecna might already have cornered the market on this one, but how about a glass (or adamantine so it won't break easily) eye? Eyes are the windows to the soul after all. *badum tss*

I think it's fun to tie the vulnerability of the phylactery to the lich's power.  For example:

- If the phylactery is vulnerable to a damage type, the lich isn't, and vice-versa. 

- The more the phylactery regularly comes into physical contact with living beings, the more undead creatures the lich can maintain in its army, so if it's a coin that's in circulation or a religious item that worshipers touch, the lich can have a huge army. 

- Anyone who learns about the phylactery, or who touches it without knowing what it is, must make an wisdom saving throw or the lich knows about them and can scry them at will.  If the adventurer receives an item that contains the information but doesn't immediately read it, the lich still has a small chance of finding out about them every time they interact with it (target gets a +10 on their saving throw).

Quote from goodreid >>

I think it's fun to tie the vulnerability of the phylactery to the lich's power.  For example:

- If the phylactery is vulnerable to a damage type, the lich isn't, and vice-versa. 

- The more the phylactery regularly comes into physical contact with living beings, the more undead creatures the lich can maintain in its army, so if it's a coin that's in circulation or a religious item that worshipers touch, the lich can have a huge army. 

- Anyone who learns about the phylactery, or who touches it without knowing what it is, must make an wisdom saving throw or the lich knows about them and can scry them at will.  If the adventurer receives an item that contains the information but doesn't immediately read it, the lich still has a small chance of finding out about them every time they interact with it (target gets a +10 on their saving throw).

 Ooo! If you combine the scrying bit, someone's earlier idea for an NPC being the phylactery, and my idea for an adamantine (disguised as glass) eye, pop that sucker into a 'helpful' NPC (maybe they don't even know someone swapped their glass eye) and the lich is literally keeping an eye on them at all times! *minor illusion badum tss followed by evil laughter*

I think Voldemort /Horcruxes was actually an excellent way to do liches:  like there could be some number of cursed objects, and they have some kind of significance to the lich

To throw it out there.....maybe a lich can be a bit more nefarious and casts Nystul's Magical Aura on the phylactery every day for 30 days and permamently changes it's aura to become "non-magical" which means spells and effects such as Detect Magic cannot find it. The Lich knows what the phylactery looks like and where it is but any would be LIch-slayers are going to have a much harder time.

I kinda assumed they were doing this anyway as a base also sticking them in custom demiplanes. A stereotypical lich has a min of 20 int and a 9th level spells at its disposal. Any tactic which includes using wizard spells to more difficult to find or kill should be just the foundation of protection. 

It makes sense that a lich would intelligently protect itself by any means that it could.  Maybe after its death it is only able to reincarnate after the phylactery is interacted with in some way, and the lich would have followers that it has trained to perform the rites?  That gives the adventurers a cult to fight

You could make it so the lichs phlyactry is something obvious, but every time after an attack hits or misses it, it teleports to a random spot in the castle and is made of a strong material so it takes like three otmr four strikes, meaning they need to be quick or they have to fight the lich again, perhaps weakened.

Hear me out,

People have an internal space... And no one said a lich can only have one phylactery... They may be able to utilize one at a time. Buuuut, if you split the soul and tag it to say... Every person in an entire town. A town, who due to their generous benefactor never grow old, never want for food due to shortage, never know poverty, work because they want to, not to make ends meet. Who know nothing but peace, happiness (actual happiness, not artificial happiness) and community spirit. And the only thing they have to so to maintain this utopia is, on the day they come of age they make the pilgrimage up the mountain to their generous benefactor to "ascend". And by all alignment checks, show in the range of good...

Let your players decide if they actually want to become the BBEG's of the story. IF they do try to kill the townsfolk, Arcane eyes in the town could show the world what the players really are...

Just saying...

Quote from Ulvebjorn >>

Hear me out,

People have an internal space... And no one said a lich can only have one phylactery... They may be able to utilize one at a time. Buuuut, if you split the soul and tag it to say... Every person in an entire town. A town, who due to their generous benefactor never grow old, never want for food due to shortage, never know poverty, work because they want to, not to make ends meet. Who know nothing but peace, happiness (actual happiness, not artificial happiness) and community spirit. And the only thing they have to so to maintain this utopia is, on the day they come of age they make the pilgrimage up the mountain to their generous benefactor to "ascend". And by all alignment checks, show in the range of good...

Let your players decide if they actually want to become the BBEG's of the story. IF they do try to kill the townsfolk, Arcane eyes in the town could show the world what the players really are...

Just saying...

 *franticly writing this down*

how would you like a job?

OHHHHHHHHH! IDEA!

you know what else has internal space?

the planet.

let your party travel around the world, making friends with diverse peoples, let them think they're the heroes. the drop the reveal. to destroy this anchent evil, who has killed thousands to keep their mockery of life going, they have to destroy the homes of their friends, their family, and everyone they've encountered on the way.

is it really worth it?

Congratulations you have figured out the original tarrask conundrum. It's an important natural part of the prime material plane 

Quote from Cleseus_Of_Castigonia >>

PLOT TWIST: The NPC that was helping them the whole way, IS the Phylactery!
This actually technically is not impossible, a phylactery can theoretically be anything...

 It turns out the true phylachtery was the friends we made along the way.

One consideration is the idea that the phylactery can be anything, but the means to destroy it is more severe than anticipated.

It might need the soul of someone pure to destroy it, killing them in the process.

It might need a true sacrifice - not a self-sacrifice, but the sacrifice of someone you love, by your hand.

It might need a worse evil to be unleashed to destroy it - EG the Tarrasque, or Tiamat.

Logically, the Lich will anticipate that the only people who would destroy it are the good guys, so they will pick something that cannot be considered good - soak the phylactery in the life-blood of a thousand innocents (consumed in the process), then quench it in a pool of gold made from melting a specific extremely religeous artefact to harden it, then use it to kill someone you love to break it.

Bonus points if, after all that, it merely serves to transfer the phylactery to the person who does the killing, perpetuating the cycle as they are now a lich. If they can overcome the temptation, they can destroy it - because only the Lich can destroy the Phylactery.

Further Issues - the Lich has to use the phylactery to kill someone they love to break it. Thus, two people (plus 1000 innocents) have to die to break the phylactery.