As an example of monster swarm, look up Angry Mob on the compendium. As a zone, it would simply be a flavor of conjuration, and they wouldn't need hit points. Instead they would be a sustain minor / as a move action, the conjuration could either move the speed of the conjurer or move 5. Alternatively, it could function as a PC spirit companion and move up to the conjurer's speed when the Lord of Sloth moves.
Perhaps someone with more monster creation knowledge could confirm whether this is a valid possibility.