Off the top of my head, I think Thanos could probably take him. But, I would have to go back and read.
Interloper is in a lot of older stories. He was a very "Odinesque" character, known for wandering and being involved in varies myths throughout human history.
Arch-Enemy: Dragon of the Moon.
Interloper had a lot of different powers and was not your standard "Eternal".
Maybe Guy could shed more light on him. I will dig up some books.
Interloper powers: Superhuman strength, telepathy, telekinese, Energy Manipulation, Psionic Capabilities:It is not known whether Interloper can psionically create illusions or manipulate atoms and molecules. He can psionically teleport himself, but prefers not to do so, finding the process physically unpleasant. Interloper can psychically induce fear in the minds of other people whom he is near. His victims need not see him in order to feel the fear he projects; in fact, he can thus so terrify them that they become too frightened to turn around and face him. He chooses to project his fear-inducing power through his cloak.
I consider him Martian Manhunter of the Eternals IMO!!