Sloth is a powerful demon from the Actraiser universe and the true boss of the first stage of the game, only through his defeat does the Master manage to free Dilligence of its evil influence and in order to do so he must transverse a dangerous maze across a river filled with demons that Sloth is using to corrupt the water.
Presumably the corrupted water of the river is what Fatigue uses in his rainfall, which in turn spreads apathy to the world of men.
Sloth takes the form of a gigantic snail that can cause earth tremors with its movements while also firing energy veams from its antennae.