When you enter Majesty's Rest, three Mad King's Guards will spawn with you. To enter The Mausoleum, you must "speak" with the Enter the Mausoleum NPC, which is located behind Rotscale. It is not necessary to kill Rotscale, however — as long as someone can reach the NPC and click through the dialogue, you will be transported into the Mausoleum. The Mausoleum itself is populated with large groups of undead that are packed fairly close together. Careful pulling should be employed to separate the groups, otherwise your party may be overwhelmed.