Splitter

Splitter is as close to an extraplanar being of the Dim planes as it's possible to be without being afflicted by difficulty staying put on the material plane. His corrupting nature undoes the usual laws of physics and additionally, has enough of a foothold in Lore powers to cause things to be "just wrong". However, he very rarely uses this power actively. Instead it is a passive effect. Without conscious intervention (or interest) on his part, around him random effects harm weaker beings, reality suffers, and things just... go very not good.

A being of immense patience and powerful intelligence, Splitter is mostly just terrifying to all he meets, but when interacted with shows himself to be an erudite and well-spoken being... despite being a mouthless entity made of razor sharp freezing crystal. He has a deep interest in the nature of conflict, particularly organised conflict, and keeps a fond eye (so to speak) on the bitter magical war between his younger brothers Medet and Niet. Most of the time, however, he roams the land, observing and gathering knowledge. Occasionally he will break into inscrutable violence, methodically destroying people or entire towns, leaving as quietly and as calmly as he arrived.

It's almost certainly for the best that, whatever he might have been planning, Splitter was not able to initiate it. In modern times Splitter is long dead, having been destroyed by a mysterious double: Splitter's weight of power on the world was so heavy that somehow a counterweight was created; an equal and opposite version of Splitter made of hot white crystal. The two immediately engaged in combat, and utterly destroyed each other. Very few records of this event exist, but hundreds of years later the yawning crater left behind remains a hotspot for minor planar activity and attracts a lot of spirits.