dmutters
commented
5 months ago Legendary monster HP, AC, damage output etc. are the same in the monster manual as non-legendary monsters.
Legendary attacks are subtracted from the overall damage budget, so the individual attacks are effectively weaker than comparable monsters. The only real differences requiring mathematical consideration are the damage from any lair actions (which I'll need to figure out a formula for).
Legendary action attacks can be handled with a note saying, essentially, "legendary action attacks are just multi-attacks that happen after other people's turns".
Legendary monsters do tend to have more resistances/immunities than other monsters, but that doesn't actually affect their HP (contrary to what the DM monster creation instructions say). The solution, here, is adding a note advising the user to "give the monster some extra resistances/immunities to fit the monster's concept".
Regeneration does affect HP. Typically, each point of regeneration per round means -3 monster HP. Damage transfer can probably be handled in the same way: estimate transfer per round, multiply by 3, and subtract that from HP. (There aren't enough monsters that do damage transfer to deduce a proper formula, so this will have to suffice.)
Possession allows a monster to effectively use an enemy's HP as its own for a while. This seems to be handled by halving HP.
I think the main challenge with implementing this will be explaining these things to the user in a concise fashion.
Lair actions Legendary resistance Legendary actions