Legendary Hatred (5e Variant Rule)
From D&D Wiki
The party is fighting the BBEG or some powerful monster! They seem to be getting the upper hand! They just dropped it to half HP...! Just what you were waiting for. The creature or BBEG roars, it now able to cast deadly spells, having higher stats, among other boosts! The PC's are now fearful as they should be!
(Note, terrible with this site, bear with it.)
But, Legendary hatred is a powerful ability to give a creature. It is a very loose and open rule that could break or boost up a monster to make it strong enough. LH (Legendary Hatred) is being built, think of what makes sense for the creature; giving a creature a high STR score when up and till this point makes not too much sense.
There should be a set percentage/HP the boss gets a buff/ability whether it be a second initiative score in the middle of the fight, or if the PC's are taking too long, then it could gain a bonus to all stats or even gain the ability for a second lair action. This is a very simple rule but could change the game if used carefully.
Suggestions for ability and how to make it.
When Boss goes to or below 50%: +2 to all stats When Boss takes Fire Damage: Becomes resistant to fire If Boss if hit by a ranged attack when wounded: Boss can catch and toss back projective If Boss is at 35%: Give [x 5-6 ability] and can use it immediately.