One of the most interesting aspects of Dungeon Crawl is the necessity to adapt to randomness. I think that this is one of the reasons that Demonspawn are such a popular race: random mutations mean a greater variety of gameplay experiences for one character. Even bad mutations can be interesting, because they force you to adopt different tactics in situations that would otherwise be trivial. So, in this vein, I've designed a species that takes randomness to the extreme by mutating every level, for good or for ill.
Note: the flavor and name of this species are placeholders, and subject to change. Suggestions are welcome.
Yeeks are said to be the missing link between goblins and slimes. These strange creatures have unstable genetics, causing them to change rapidly as they grow. Yeeks gain a random mutation every XL, and start with evolution 2 as a permanent innate ability. Yeeks have the potential to be very powerful, or very weak. Adapting to your good and bad mutations is necessary for survival.
Furthermore, yeeks are able to split and create smaller versions of themselves, called yeeklings. These are bizarre masses of flesh and slime that possess most of the traits of yeeks, and scale with the yeek's level. If possible, they will also retain the yeek's mutations. Yeeklings are not a replacement for summons. Instead, they are designed to give more options to melee focused characters, and can supplement a summoning playstyle with permanent minions. Other traits of yeeklings are tbd. They might be able to cast some of your spells, or even split into yeeklinglings.
Starting at level 7, you can create one yeekling by sacrificing a portion of your permanent HP. The yeekling will last until it dies or until you dismiss it. When the yeekling disappears, you slowly gain your max HP back. At level 14, you can sacrifice additional HP to create a second yeekling, and create a third at XL 21.
Unfortunately, yeeks are weak and bad at most skills. In the early game, their only saving grace is their high stealth and fast growth, which means they gain levels more quickly than even humans. They also emit a film of slime that covers their equipment, neutralizing acids and quickly healing wounds.
The yeeklings were born from the thought “What would a slime race do? They would split, of course.” The function of the yeeklings would depend on what your character does. If possible I'd like them to scale on your skills, health and damage: a fighter would create tough yeeklings that improve your damage output and tank, a stabber would create dodgy yeeklings that give more stabbing opportunities by distracting enemies, and mages would create blasting yeeklings that blow things up. Think of them as mini-mini Mara clones.
A possible additional ability for yeeklings is “Lose two mutations, gain one” as an activated ability. This would make it possible to remove aggravating or game-ending mutations at the cost of possibly losing your good mutations.
I haven't playtested this species with the “One mutation per XL” benefit yet, so these observations are somewhat conjecture, but also come from experience playing with Xom and Jiyva. Yeeks will be VERY weak in the early game, being unable to run from foes and relying only on stealth. They will also tend be very powerful if you survive the midgame, due to selection bias and the abundance of good mutations. Yeeks are to species what chaos knights are to classes: probably not viable in the tournament sense, but also lots of fun.