My group is playing through kingmaker, and we just had a couple of slightly anti-climactic fight against regenerating monsters. I've written up some new rules to make regeneration a little more scary.
Regeneration is meant to represent a creature that just won’t die: the troll that keeps getting back up, or the hydra that grows heads faster than you can hack them off. In these stories, regenerators frequently keep getting back up until they are totally burned, melted, frozen, etc...
In 3.5, this was represented by needing to kill the regenerator entirely with whatever bypassed their regeneration. I never found this satisfying because then martials have nothing to do.
Pathfinder's solution is worse, since now hitting a troll with a single point of splash damage negates the whole ability.
We increase the damage needed to break regeneration. Our goal is that a CR appropriate encounter should require the party to make some extra effort to overcome regeneration.
Breaking Damage: Any damage source that deals any amount of damage that bypasses regeneration counts as breaking damage.
Regeneration Threshold: A creature's regeneration threshold is equal to
When a creature takes an amount of breaking damage in excess of its regeneration threshold its regeneration turns off for one round.
Get back up: if a creature with regeneration is reduced below 0 hp, it stops gaining hit points for one round (this is not regeneration turning off). At the start of its next round, it heals enough hp to put it at positive hp equal to 4 times its regeneration value.
On diehard and ferocity: run.
On energy resistance and DR: Even if a creature has enough resistance/DR to totally negate the portion of the attack that it is vulnerable to, the attack still counts as breaking damage. Otherwise a troll with resist energy is basically unkillable by a martial with a flaming weapon.
My goal is that if a party only has a single source of breaking damage, they should have about a 50% chance to stop the regeneration of a creature non boss creature.
Trolls have a threshold of 21. A level 7 unoptimized caster does 21 damage, and 10 on a save. From experience, our martials put out 20+ damage a round on a good round.
How will this rule work at higher levels? Is there something i'm not seeing?