Valaskjalf Dungeons
A number of high profile figures have been imprisoned there, including the Enchantress, Lorelei, Sigurd the Everglorious, Loki, and Algrim the Kursed.
During the Second Dark Elf Conflict, Algrim was responsible for a major prison-break, which released an estimate 138 prisoners, primarily members of the Marauder Army. After Loki was returned to Asgard after the War of the Worthy, Frigga recruited him to act as her agent, tracking down and returning the escapees.