The Temple of N'cthail is located in the water west of Vissidal Island, at 79.7N 57.4E. There is a portal a short run north of the Bandit Castle, at 70.4N 50.5E, restricted to characters level 100 or greater.
The temple is part of the Helm of Isin Dule Quest, and has portals to the Eastern Temple Catacombs, Western Temple Catacombs, Northern Temple Catacombs, and the Vault of Eternal Slumber.