User can regenerate from even the most serious of wounds at the cost of mutating after each regeneration. The user could go insane from the reconfiguration of their physical form. The user may also become more resilient from whatever wounds they heal and mutate from.
After suffering brutal experimentation, Killer Croc (Batman: Arkham/DC Comics) mutated even further upon regenerating.
As a result of his unstable body, Fusion Zamasu (Dragon Ball Super) mutates into a purple monster upon regenerating himself.
As William became increasingly injured during his fights with Leon S. Kennedy and Claire Redfield, the G-virus rapidly healed and evolved his body to that a clawed juggernaut...
...In his third form, the G-virus mutated William in a hulking, multi-armed brute with his humanity all but lost...
...At this point in his forth form, William had already sustained a lot of damage, forcing the G-virus to compensate mutate him into a ferocious monster of claws and multiples eyes...
In his fifth and final form, William has sustained significant damage to the point where the G-virus was unable to fully heal him, leaving him to become nothing more than a mindless mass of flesh and teeth.
Aoba Takatsu's subordinates (Code:Breaker) possess the ability Cell Stimulation, which causes their cells to grow and regenerate whenever they are attacked, mutating them into monstrosities.
After losing an arm to a demon in limbo Victor Borkoski a.k.a. Anole (Marvel) grew a brand new limb more powerful than the last.
Crawler's (Worm) regeneration eventually turned his body into a monstrous form.