To enter the Prism of Light you must first purchase a ticket for 10000 gold from the NPC who can be found just outside the dungeon.

To gain access to the Peerless boss, the Shimmering Effusion, you must gather a number of items from the creatures in the dungeon and drop them on the pillars in the room with the Unfrozen Mummies. This will result in a number of teleport items that will take you to the peerless area.

If you wish to exit the dungeon you can double click your ticket.