Stygilax

Actions

-Hellfang: Upon joining a battle, the Stygilax will inmediately choose a random NPC/monster target, move towards it and restrain it for up to 3 rounds if it's Lvl 1 or 5 rounds if it's Lvl 2. In each round, the victim has the choice to spend its turnattempt a Strenght (Athletics) contest against the Stygilax's Strenght: if it suceeds, it will be freed; if it fails or chooses to not contest, it will keep restrained until it wins a contest or is freed in the 3rd/5th round. The moment the victim is freed, the Stygilax will flee (contest won) or be consumed (final round), and the victim will suffer the Incapacitated condition for the rest of the round. When the next round starts, the victim will either burst in flames and instantly die or transform into a Descended version of itself (see individual statblocks) with the same level as the round number in which the Stygilax left, and if the Stygilax was consumed, it will reappear when its defeated and suffer from the Incapacitated condition for the following two rounds, then flee (if it survives).
OPTIONAL RULE: During the round in which the victim is Incapacitated, hitting it with 17 or more Radiant damage before the end of the round will stop the Descension and, if the Stygilax was consumed, it will be destroyed.

Regional Effects

-Reality Warping: While the Stygilax is not Stunned, all PCs and NPCs/monsters within 3 miles have disadvantage in Wisdom rolls and will suffer from hallucinations and other illusionary effects to the GM's choice.

"Have never encountered a Stygilax myself; guess I'm quite lucky. But I've heard of them, and I wish I never did. Don't let their simple canine-like appearance and those goofishly long saberteeth deceive you: these creatures aren't your average wild dogs. They are said to be Lenopharia, brainless entities made of Essence; in our monster's case, the most destructive known type of Essence: the Demonic Energy. And they don't like keeping it for themselves: instead, they will chase anything that moves in order to share its destructive power. When its fangs finally get to pierce the flesh of the unfortunate victim, the Stygilax will inject itself into its body, and then one of two things can happen: it dies a painful death, or suffers the entire supernatural process known as Descension and eventually sees itself reborn, rising up as a more powerful incarnation of itself, controlled by the Essence beast and ready to destroy anything on its path.   Entire worlds have succumbed to the most powerful Stygilax hosts, but they have two weaknesses. See that bulge on the Descended thing's chest? That's a Dark Heart, and if you manage to destroy it, the thing will lose its powers and fall dead, releasing the Stygilax's Essence and allowing it to be destroyed. Of course, if the victim is a loved one or simply something you're not interested in destroying, there's a less murderous way to stop it: using other kinds of "good magic", which not only can "cure" the Descension entirely, but is one of the few known ways to destroy the Stygilax itself. But heh, good luck obtaining that power. Thankfully, I doubt we'll be unlucky enough to encounter one of those... Hope so."

Might be found anywhere.