Creature Abilities
Source Alien Core pg. 208
The following creature abilities are listed here because they are shared by many creatures or are highly complex. The statistics for individual creatures might alter the traits, the number of actions, or other rules of these abilities. Anything noted in a specific creature's stat block overrides the general rules for the ability below. In these abilities, “monster” is used for the creature that has the ability, to differentiate it from any other creatures the ability might affect.
Regeneration
Source Alien Core pg. 210
This monster regains the listed number of Hit Points each round at the beginning of its turn. Its dying condition never increases beyond dying 3 as long as its regeneration is active. However, if it takes damage of a type listed in the regeneration entry, its regeneration deactivates until the end of its next turn. Deactivate the regeneration before applying any damage of a listed type, since that damage might kill the monster by bringing it to dying 4.