In Demon's Souls players were able to invade each others game as a Black Phantom, to hunt down the other player and try to kill them. In spiritual sequel Dark Souls, the developers have a more ingenious way to harass others.
The Pyromancer has the ability to summon a Gravelord, setting it free upon another player's world to attack and harass that player until it's killed. The Pyromancer can't decide who it is they're attacking, they won't even be told how the attack goes. So why do it? Because, From Software thinks, sometimes it's fun to be a little malicious, a little tormenting to a stranger.