In this story i'm working on, it's only called:

"The Corruption" (bumbumbum)

Basically its the corporeal embodiment of pure chaotic evil that has the potential at times to be sentient, and infinitely intelligent given the proper resources. Those resources? Everything in existence.

Noone knows where it came from and it's sole existence is to live up to its name, "corrupt" the fuck out of everything.

Timeline-wise, initially an extremely powerful omni-universal race basically commits to Mutually Assured Destruction on everything that ever was, in an attempt to wipe out The Corruption. It works but minuscule smidges of it are still prevalent in small parts of reality.

Fast-Forward eons later to a world similar to ours, only 11 times bigger and under a global totalitarian "republic" possessing technology on par with a near-future world. An expedition of scientists and soldiers is sent out to explore a newly discovered island off the coast of a far away and sparsely charted dark continent for "Classified purposes".

There, beneath the dark canopies of the dense foliage is an impossibly vast and imposing ruined temple complex of great stones and boulders. The expedition unleashes the corruption after fuddling around in an untouched pyramid at the heart of the complex, and it quickly consumes the entire expedition with its one directive in mind, to corrupt everything.

The corruption can manifest in a wide variety of forms but in this world, prefers flesh thanks to its widespread availability, malleability and usefulness as a binding agent for more durable materials.

In combat, The corruption will utilize the flesh of its hosts and "recast" it into stronger and more fighting efficient forms, utilizing materials as well such as metal, stone, or wood for ablative armor or to enhance their internal structure.

Presently, the corruption has been thankfully quarantined on the island thanks to their inability and lack of intelligence to pilot ships.