Kill them and loot the nearby Alchemist's Chest. On arrival at the ritual site, the leveled Necromancers will attack on sight, along with Selonius the Apothecary who leads the group. The Ritual Site can be found on the plateau North of the Sundered Towers. This quest begins immediately after reading Antioch's Journal, Volume 2 in Ravenscorn Spire. I should investigate this.' ―the Dragonborn's Journal Objectives He has already departed for the Dragontail Mountains in search of something, but has sent one of his followers to conduct some sort of Alchemical Ritual on a plateau overlooking the Reach. 'The Necromancer's leader, Antioch, is seeking to use a Tome stolen from the Vigil of Stendarr to become an undead Lich.
