Malum

Malum is an Immortal, who usually resided in the Etheral Plane, with a collection of artifact from all kinds of different worlds. He's actually an Immortal from the BECMI/Cyclopedia version of D&D. He's searched for the Scronching Blade and the Octahedron of Transcendence. He joined the Nightmares after being put under Shackles of Servitude. Even still, we know he was evil even before then, which is where he got his name from.

We also know that Malum isn't truly on the Nightmares side-He plans on being the Absolute God of Reality himself, unleashing the Vegetal.. Betraying the Nightmares when the time arrives. He is in fact the person in Sans' second vision.