The True Tomb is marked near its entrance with a symbol noted from the Arcane Sanctuary and has an Orifice to place the Horadric Staff which opens a path to Tal Rasha's Chamber. The other tombs are a good source of loot, so they are also worth exploring. Placing the Horadric Staff inside the orifice opens the way to the final zone of Act II, namely Tal Rasha's Chamber.