The Horus Heresy: Book Five - Tempest is the fifth book released as part of Forge World's Horus Heresy Series of campaign and army books that takes place during the Horus Heresy. The book deals with the Calth Atrocity, where the Word Bearers Legion launched a devestating suprise attack on the Ultramarines Legion. The book contains rules and special characters for the pre-Heresy Word Bearers and Ultramarines Legions, the Imperialis Militia, and Chaos Cults. Included within is the Imperialis Militia & Cults Army list, and an updated Space Marine Crusade Legion army list, as well as an updated Mechanicum army list.