Karla-Gard

In a parallel universe, the Karla-Gard were humanoid fungal beings with a collective consciousness that originally grew from "the rot of the [Time] Vortex" — specifically, the remains of Beasts of Time, Reapers and Vortisaurs — with the ability to "merge" with other life forms, taking over.

A group of Karla-Gard followed this universe's River Song's TARDIS across the Void into the Doctor's universe, intent on revenge after she "swiped their Spore Queen" in order to develop a gas which would kill them (creating a causal loop, as they were already pursuing her for this reason). (AUDIO: The Two Rivers)