Board Thread:Dark Souls/@comment-59.101.80.154-20161007133514/@comment-4511886-20161007150039

A player that was part of the Gravelord Servant Covenant infected your world using an Eye of Death, and then died in some way. This would, in the event that you're in NG+ (which means you've beaten the game at least once with that character) cause additional enemies, in the form of black phantom versions of normal enemies, to appear in the infected area, which are far stronger than their normal counterpart. If you're not in NG+, it merely allows you to invade the Gravelord Servant to fight him.

TL;DR: Another player used an online play item that randomly chose you as its target, and then the player died.