Located at -96, 60, -903 in Everfrost. It is smallish but has several mini-bosses, including one epic level 44 valkyrie.
Originally you gained access by completing the five gravestone quests from an island near docks in Everfrost. After doing this Karg Icebear spawns. Hail him to complete the quest and gain access. All access quests were made optional some time ago.
If you head inside and clear the dungeon, Karg will spawn again. You can then speak to him to finish another speaking only quest and receive a reward.