As mentioned in past questions, in my world, there is monsters-creatures mutated by Chaos Energy. My characters will be facing monsters, which is fine-so far, so good, right? No trouble there. However, problems automatically arise when in a world where magic plays by rules, just like the rest of nature, a monster inexplicably defies death or injury, taking on a different form to continue attacking the party.
Example of Different Forms: Let's say there are two adventurers (Bob and Alice) facing a group of Chompers (the lead of which is named Eggbert, which the adventurers are unaware of because they didn't think to ask). After Bob and Alice wipe out the rest of Eggbert's comrades, Bob lands a critical on Eggbert, which should have K'Od him, shattered his shell, and crushed his insides.
Instead, Eggbert transformed; somehow, drawing upon willpower and ambient magic, he managed to endure as two different beings: Shelly, a cloak-like monstrosity formed of his shattered shell and shell membrane, and Egolk, a gumdrop-shaped blob of translucent white with a toothless mouth, two suction cup-tipped tentacles (formerly Eggbert's legs), and a yellow core that serves as his stomach and who knows what else.
Now Alice and Bob are, to put it lightly, screwed. Shelly may be fragile, but he's also as slippery and quick as any snake, with razor-sharp reflexes and the capability to shred his enemies with the "scales" all over his exterior. Meanwhile, Egolk may be fleshy, but with his bouncy-ball-esque mobility, his ability to engulf edible items and creatures for extra HP (which Shelly shares), and his surprisingly powerful tentacle attacks, he's no pushover either. And to make it worse, if Alice and Bob don't keep them separated, they'll fuse into a stronger Eggbert.
Alice, as the party Assassin, is the fast one, so she tries to take on Shelly. Bob, as the Tank, elects to not only separate Egolk from Shelly but to smash him so he can't fuse, period. Alice tries to take Shelly down with her shuriken, but they fail to penetrate Shelly's scales. Alice then comes after Shelly with her dagger and ends up having to climb a tree, and while she's distracted, Shelly comes down and engulfs her head.
Bob notices this and goes to help, but then Egolk strikes from behind and engulfs Bob's head. The result? Total Party Kill. The concept isn't the problem, explaining it is. In a world that plays by natural laws, of magic or nature, instead of gaming conventions, this kind of thing seems to demand an explanation. So my question is, how could monsters shift forms during combat?
Additional Examples to Aid Answerers:
When an insect monster is at max HP when struck by a Critical Blow, especially one in a cocoon or egg, a Hateful Husk may result; the shell, egg, or cocoon given a life of its own by the essence of its owner left inside it. Soulful Shells, the Dropped shell of an insect possessed by its lingering spirit, may occur when the insect was KOd in one hit.
When a Crawling Eyeball is struck by a Critical blow, its eye may be destroyed, but there is a chance of it popping out and entering the battle as an Enduring Eyeball. Something similar can happen with Plops and their eyestalks.
When a Rungi or Wild Anklebiter is beheaded, the result is either a Confused Cap or Forlorn Flower. In both cases, the monster's body will remain as a new monster, like a Seeking Speeder or Stained Stem. Other monsters do this as well, but those are the best known for doing this.
Monsters that exhibit chimerism may split in combat, like a Lamia or Chimera. In the former's case, the result will be a Mesmerizing Maiden ( a Lamia's upper body but w/legs) and a Headless Horror (a headless snake with a leech-like mouth). In the latter's case, you'd get a Flailing Viper (the tail), a Gnashing Goat (the goat head w/body), and a Lost Lion (it looks lost because it's missing pieces of itself, which confuses it). Chompers do something similar in that they may split their essence between two different parts of their body when struck by a felling blow.
In extreme (and rare) cases, monsters may marshall their will to stay on this plane; like a troll with full health, being dismembered only to reassemble and seek revenge, a giant cockroach losing its head only to replace it with another's, or a gargoyle somehow patching its broken body up with slime instead of dying.
I could go on, but basically these monsters, unlike other members of their kind, manage to endure on the battlefield. It's not understood why; usually, a monster is killed and only a Remnant, a magical shadow of its being, remains with its Drops. The injuries these monsters take before transforming should result in death of either the whole or part of them, yet somehow these monsters or monster parts anchor and stabilize themselves to endure and fight on. How is this possible?
Seriously, will alone cannot anchor the soul to this world, and while magic should, through its linking body and soul together, be able to do this, we don't understand how some monsters manage it while others don't, especially considering these monsters exhibit no extraordinary regenerative ability aside from this and that these same monsters are usually low on vitality, willpower, and mana-magical energy-before apparently expending it to transform in this way.
If additional clarification is needed, please let me know, I appreciate feedback! You may find these helpful: