I know that there is (or at least, used to be) one portal in the dungeons under Dragonspear Castle that linked the castle to Avernus, the first level of the Nine Hells. Are there other such fixed portals documented in the Forgotten Realms that connect to the Nine Hells? (Sources can be from any Edition of D&D, or from other official publications like Dragon Magazine or novels).
- Fields of the Dead
According to Elminster's Candlekeep Companion (p46-48):
the best way to enter Avernus is via the original gate the Hellriders used
and that the hellgate is
on the Fields of the Dead.
Descent into Avernus also mentions a gate in the Fields of the Dead, but doesn't provide any more specifics.
- Dragonspear Castle
The castle was infamous for the inter-planar portal located in the dungeon beneath its keep. This portal linked the castle to Avernus, the first level of the Nine Hells, allowing passageway for baatezu and other devils into Toril.
- Myth Drannor
A portal to Avernus was erected in the spring of the Year of the Bloodbird, 1346 DR in the Burial Glen of Myth Drannor (...) interference from Malkizid caused it to become permanent. The portal was eventually closed by the Knights of Myth Drannor in 1357 DR (...) [Dragon 359 pp.102-103]