...
Serpents in the rooms are on a cycle with the other types of spawns, simply pick a room and work that spawn if you want those. For the ones in the room with the teleporter back to the beginning of the Tomb, someone probably lead them off either closer to the fire/energy wall at the front, the bridge to the Abyss in the back or down the side path towards Nipralem (or however it's spelled). Killing them will reset their spawn in the open area.