They appear during the Eye of the Vortex campaign when a High Elf faction begins a Ritual. They will spawn near a ritual site and will attempt to raze it, attacking anything else in their way. They are automatically at war with the High Elf faction that started the ritual.
Other Ritual Intervention Chaos Armies include:
They were also involved with the Chaos Invasion in Mortal Empires where they will spawn in the sea to harass the factions introduced in Total War: Warhammer II. They were removed from the Chaos Invasion in December 2020.