I didn't think such exist until I began reading one.
Synopsis: Three Russian self-inserts into the Mass Effect universe. One as a Tyranid, one as a Primal Zerg and one as a Xenomorph (already almost too scary to read) Tearing through Omega at first, they join forces thanks to the Tyranid telepathy, find themselves a largely uninhabited planet then begin making a joint Swarm combining the strong sides of their respective species. Zerg eating Krogan masters of war and Asari commandoes to incorporate their strengths into their genome, for example.
Then they begin saving the galaxy from the reapers by converting its denizen into a part of the Swarm.
I stopped reading in terror when Shepard met Subject Zero (Jack) transformed like Kerrigan was in the Star Craft canon -- after the Normandy had barely escaped Omega overrun by mutant fanatics who we are shown beating Aria and injecting her with a mind control parasite.
I could only guess it continues in the vein of "and then things got worse".