Greetings! Hello all! First time poster, probably the first of many in all honesty.
Background So first and foremost, the large campaign I'm running at the moment with 7+ players at my table is atypical from your usual campaign as most of the party are monsters of different varieties (all of them are more or less around level 8/9). As such a lot is being houseruled/homebrewed to accommodate certain things. All the players are (or have become) evil in alignment, though there is still, realistically speaking, conflicts that would arise between their characters just from the blurbs given in various monster manuals that have been trawled through. However, there is a binding force keeping them united as they inflict death, terror, and ill-fortune upon virtually anyone they come across. So for now, let's ignore the "why would these characters be together?" because that is a long story.
Situation So *inhales* three of my players elected to assault an escorted caravan when they were warned there might be an archmage in the mix. Naturally all three met terrible magical ends. So consequently the three make new characters, one of whom is a dragonborn (black, naturally) anti-paladin (not the plague bearing kind). Said dragonborn wants to bring about the demise of a very powerful monotheistic order and in a sense, murder a god. But I digress...
One of the group (I'm refraining from calling them a party because it really doesn't do justice to the injustices they're inflicting) is a lamia (not the snake thing, the sphinx thing). Lore-wise lamias are really really vain and hedonistic, self-centred, manipulative, and immortal (age-wise). What the two players want to do is have the dragonborn antipaladin (who yes, wears full plate of course) ride the lamia like a mount into combat to utilise all the anti-paladin's mounted combat features while allowing the lamia to continue performing actions and spellcasting as per usual.
My assessment (A "+" indicates a reason for, a "-" indicates a reason against, three marks indicate a strong reason)
+ I like to be a fun GM, DnD is all about the players having fun within rules that can be adjusted to maximise fun. --- The setting is quite a grim one (mainly due to player actions), trying to have the players be conscious of morality, and silly things always snowball. +++ Player creativity is important to me. They have instigated and performed false flag operations, counter-intelligence operations, founded a cult in a major city, had a oblex running an extremely successful alchemy workshop, usurped three neighbouring lords simultaneously, etc. I don't want to stifle their will to be tenacious and inventive. + They plan to roleplay it out. The lamia starts reluctant, then accepts the advantages. - They plan to roleplay it out. The lamia is vain, would she truly allow herself to be a mere steed when she has commanded the downfall of an empire (backstory)? Furthermore this dragonborn anti-paladin is a newcomer, not even the same pseudo-military rank (within the evil hierarchy). - The dragonborn is over 7ft tall, nearly 200lb, and wearing heavy armour. If I were to allow it at all, there would be a movement penalty at least to the lamia, right? + Lamiae do have a natural strength score of 16. --- The heavy armour argument. The lamia (I think I allowed her to use a breastplate, medium armour, with proficiency so she could spellcast). However, she is certainly not proficient in heavy armour.
If you wear armour that you lack proficiency with, you have disadvantage on any ability check, saving throw, or attack roll that involves Strength or Dexterity, and you can’t cast spells.
(Player's Handbook, page 144) I would argue that carrying someone 200lb plus plate armour would trigger this too.
- Momentum surely should carry some influence on mounted attacks, and momentum is influenced by speed. Riding horses have 60ft of movement as opposed to a lamia's 30ft. + Use of a saddle. Theoretically one could ride a lion if one fitted it with a saddle. And not get mauled to death by it... + Anti-paladin does have the Mounted Combat feature. - Anti-paladin does have access to the Find Steed spell.
TLDR (I don't really blame you): Should a dragonborn in full plate be able to mount an allied lamia (for beneficial combat purposes)?
Look into other posts and discussions on using a PC centaur as a mount. The problem (and I only say that from an applying rules standpoint) is that you are allowing a PC to play a large creature. When the centaur was added as an official PC to 5e it was made a medium sized creature, but centaur as a monster is still large. (Same with minotaur) You mention the natural strength of a lamia, but what is the character's actual strength? Did they roll a 16? Are you using encumbrance? (The variant rules) Or are they just using the monster stat block?
As for a saddle, in 5e you don't need a saddle for a mount. It helps you from falling off though. Mounted combatant feat is for a large mount, not a medium PC (the centaur as a mount discussions). Also, a "mount" listed in the rules is not interchangeable with a player character, so mounted combatant wouldn't apply to being on another PC's back. A large/monster lamia would have barding instead of armor. (Unless you've made it a medium PC)
Basically, you've traveled far enough down the homebrew rabbit hole that any ruling you give will be perfectly fine. No need to try and shoehorn in rules for it at this point.
Assuming he is the lamia as is from the monster manual, he can only carry 480 lb (link below). More than that, and their movement is reduced to 5 feet (over 960 lb, speed is 0). Encumberance is not tied to spellcasting or armor proficiencies (you can carry any and every type of armor withput nonproficiency penalties). Between the rider and the 2 armors, that is 305 lb, so their weapons, shields, food, water, clothes, ammo, magic items, tools, saddles, packs, etc cannot weigh more than 175 lb to move effectively while mounted.
Greetings!
Hello all! First time poster, probably the first of many in all honesty.
Background
So first and foremost, the large campaign I'm running at the moment with 7+ players at my table is atypical from your usual campaign as most of the party are monsters of different varieties (all of them are more or less around level 8/9). As such a lot is being houseruled/homebrewed to accommodate certain things. All the players are (or have become) evil in alignment, though there is still, realistically speaking, conflicts that would arise between their characters just from the blurbs given in various monster manuals that have been trawled through. However, there is a binding force keeping them united as they inflict death, terror, and ill-fortune upon virtually anyone they come across. So for now, let's ignore the "why would these characters be together?" because that is a long story.
Situation
So *inhales* three of my players elected to assault an escorted caravan when they were warned there might be an archmage in the mix. Naturally all three met terrible magical ends. So consequently the three make new characters, one of whom is a dragonborn (black, naturally) anti-paladin (not the plague bearing kind). Said dragonborn wants to bring about the demise of a very powerful monotheistic order and in a sense, murder a god. But I digress...
One of the group (I'm refraining from calling them a party because it really doesn't do justice to the injustices they're inflicting) is a lamia (not the snake thing, the sphinx thing). Lore-wise lamias are really really vain and hedonistic, self-centred, manipulative, and immortal (age-wise). What the two players want to do is have the dragonborn antipaladin (who yes, wears full plate of course) ride the lamia like a mount into combat to utilise all the anti-paladin's mounted combat features while allowing the lamia to continue performing actions and spellcasting as per usual.
My assessment
(A "+" indicates a reason for, a "-" indicates a reason against, three marks indicate a strong reason)
+ I like to be a fun GM, DnD is all about the players having fun within rules that can be adjusted to maximise fun.
--- The setting is quite a grim one (mainly due to player actions), trying to have the players be conscious of morality, and silly things always snowball.
+++ Player creativity is important to me. They have instigated and performed false flag operations, counter-intelligence operations, founded a cult in a major city, had a oblex running an extremely successful alchemy workshop, usurped three neighbouring lords simultaneously, etc. I don't want to stifle their will to be tenacious and inventive.
+ They plan to roleplay it out. The lamia starts reluctant, then accepts the advantages.
- They plan to roleplay it out. The lamia is vain, would she truly allow herself to be a mere steed when she has commanded the downfall of an empire (backstory)? Furthermore this dragonborn anti-paladin is a newcomer, not even the same pseudo-military rank (within the evil hierarchy).
- The dragonborn is over 7ft tall, nearly 200lb, and wearing heavy armour. If I were to allow it at all, there would be a movement penalty at least to the lamia, right?
+ Lamiae do have a natural strength score of 16.
--- The heavy armour argument. The lamia (I think I allowed her to use a breastplate, medium armour, with proficiency so she could spellcast). However, she is certainly not proficient in heavy armour.
(Player's Handbook, page 144)
I would argue that carrying someone 200lb plus plate armour would trigger this too.
- Momentum surely should carry some influence on mounted attacks, and momentum is influenced by speed. Riding horses have 60ft of movement as opposed to a lamia's 30ft.
+ Use of a saddle. Theoretically one could ride a lion if one fitted it with a saddle. And not get mauled to death by it...
+ Anti-paladin does have the Mounted Combat feature.
- Anti-paladin does have access to the Find Steed spell.
TLDR (I don't really blame you):
Should a dragonborn in full plate be able to mount an allied lamia (for beneficial combat purposes)?
Look into other posts and discussions on using a PC centaur as a mount. The problem (and I only say that from an applying rules standpoint) is that you are allowing a PC to play a large creature. When the centaur was added as an official PC to 5e it was made a medium sized creature, but centaur as a monster is still large. (Same with minotaur) You mention the natural strength of a lamia, but what is the character's actual strength? Did they roll a 16? Are you using encumbrance? (The variant rules) Or are they just using the monster stat block?
As for a saddle, in 5e you don't need a saddle for a mount. It helps you from falling off though. Mounted combatant feat is for a large mount, not a medium PC (the centaur as a mount discussions). Also, a "mount" listed in the rules is not interchangeable with a player character, so mounted combatant wouldn't apply to being on another PC's back. A large/monster lamia would have barding instead of armor. (Unless you've made it a medium PC)
Basically, you've traveled far enough down the homebrew rabbit hole that any ruling you give will be perfectly fine. No need to try and shoehorn in rules for it at this point.
Assuming he is the lamia as is from the monster manual, he can only carry 480 lb (link below). More than that, and their movement is reduced to 5 feet (over 960 lb, speed is 0). Encumberance is not tied to spellcasting or armor proficiencies (you can carry any and every type of armor withput nonproficiency penalties). Between the rider and the 2 armors, that is 305 lb, so their weapons, shields, food, water, clothes, ammo, magic items, tools, saddles, packs, etc cannot weigh more than 175 lb to move effectively while mounted.
https://www.dndbeyond.com/compendium/rules/basic-rules/using-ability-scores#Strength