Planar Bestiary
These monstrosities are every bit as dangerous and devious as they look. Enhancing their natural acid.producing capacity with magical runes, a grethk nest can rapidly render a region uninhabitable by anything other than themselves and their teeming young, which require a strongly acidic environment to thrive. If left unchecked, they can overrun an entire world.
The grethk shared their homeworld with a species known as ovisans. The two warring species developed together over time, stealing rune "technology" from each other in their rivalry, and devising other attacks to exploit their opponents' weaknesses.
Ovisans eventually got the upper hand, destroying almost all the grethk. But hundreds of fertile adults used rune magic to flee and hide in other worlds of the Material Plane, intending to establish new nests. Ovisan hunters now seek those nests wherever they spring up, vowing that no world shall fall to grethk overpopulation.
Grethk are fecund and reproduce like fish in that they lay a great quantity of eggs. Without natural predators in the acidic environment the eggs need to come to term, grethk can quickly overpopulate a region, as newly hatched grethk are force-grown to adult status within a few days thanks to rune magic. Thankfully, a given grethk lays eggs only once in their lifespan.
Ovisan hunter, page 101
Grethk can breathe air and water.
GM Intrusion: A rune glows on the grethk's shell, and a character is blinded with a film of acid. Each round a character is blinded from this effect, they take 1 point of damage (ignores Armor) until an action is taken to pour water or other liquid into the character's eyes to wash out the acid.
Null field, page 393
Motive: Establish colonies, defense
Environment: Aquatic locations on worlds of the Material Plane, alone or building up a nest with two or more others
Health: 15
Damage Inflicted: Moderate wound (5 points)
Armor: 3
Movement: Short; long when swimming
Modifications: Speed defense as level 4 due to large size; resists magical attacks as level 7; perception as level 3 due to weak eyes
Combat: The grethk attacks twice each round with their pincers.
About three times a day they can activate a magic rune to conjure an acid burst targeting up to three creatures within short range, inflicting 8 points of damage on a failed Speed defense task, or 2 points even if successful.
The grethk can activate a different magic rune about three times a day to conjure an oily acid slick within short range, filling an area an immediate distance in diameter. Each creature in (or that later enters) the acid-slicked area that fails a Speed defense task takes 5 points of damage and falls prone; if successful, they take 2 points of damage but don't fall. Prone targets must succeed on a Speed task as their action to crawl out of the area. Grethk, however, can maneuver within the area with impunity. The slick persists for ten minutes.
A grethk is immune to acid attacks but vulnerable to electricity; every time they take 1 point of electricity damage, they take 1 additional minor wound (1 point).
Interaction: Grethk normally speak only their own language. Even if somehow contacted, they are highly territorial, and prefer to think of all other creatures, including intelligent ones, as a food source or competition for resources that should be eliminated.
Use: Something is killing game in the noble's forest, and they're looking for someone to go in and find out what (after all their own underlings failed to return from a similar hunt).
Loot: Someone who spends a few minutes studying the runes covering a defeated grethk's shell and who succeeds on a magic lore task could salvage a runeplate null field cypher that provides acid protection.