Stepping into a ring is handled at
Process_StepIntoRing. In the Crystal Cavern it takes
you to room 57; anywhere else it takes you to room 43, unless bit 5 of
Flag_QuestProgress is set, in which case it takes you to room 56, which is the end of
the game, written as a room description. That bit is set when the urn is
thrown into the fire at
Process_ThrowUrnIntoFire, which also kills the Fomorian tribe.