The Slumbering Sanctuary
is located after beating Conjunctivitis in the Insufferable Crypt
. You access the Insufferable Crypt
from the Ancient Sewers
. The Ram Rune
is required to get there.
For your first visit you will follow this route:
Prisoners' Quarters > Toxic Sewers > Ancient Sewers > Insufferable Crypt > Slumbering Sanctuary
I found this map to be particularly helpful.