Maitimírë is an Underdark lake of Ufthag. It is on the northern side of Cairn Ascent, requiring a trek around this road to surface or by going up river from the Green Nebulous.
Periodically an inter-system gateway opens inside Maitimírë; linking Bal-Kriav with one of the worlds of the Gray Waste. This gateway is the Styx Fountain, and when it opens a tributary of the river Styx bleeds into it. The waters of Styx taints the water it touches causing some of the stone in the lake's bottom and sides to turn into Styx Granite.
Maitimírë and the caverns around it are often patrolled by the Tîra-Phâth. These cultists are not so interested in travelers of this realm, but in threats posed by those that come out of the Styx Fountain.