Xancrown's Prison

From the introduction:

Most of the background for Xancrown’s Prison is contained in The Demonplague campaign’s first three modules, The Frozen Necromancer, The Winter Druid’s Legacy, and Icefall. Xancrown’s Prison picks up directly where the last adventure left off.

If the characters have come this far, it is assumed they are looking to enter the Demonplague Dungeon to end Xancrown’s reign of terror once and for all using the Minas Herion ritual. The characters should have all the components of the ritual and have learned to cast it from Feriae or one of the other Minas Herion druids.