Zanthus was a Chapter Master in the Dark Angels Legion's Third Order, which was led by Grand Master Haradin, during the Horus Heresy.
In the aftermath of the Battle of Perditus, the Third Order was assigned by their Primarch Lion El'Jonson to a strike force, led by Seneschal Corswain, that was charged with both hunting down the fleeing Death Guard forces of Captain Typhus and to bring word of the battle to the Space Wolves Legion's Primarch Leman Russ. However after many months, the strike force could not find any trace of Leman Russ and as for Typhus, they found only worlds that were left in disarray or that had been destroyed by his touch. During one of the many war councils between the strike force's commanders, unrest began to show about their lack of progress. After much arguing, it was finally decided that the strike force would be split off by Orders; each with their own goals to pursue. Corswain would go with Chapter Master Belath's Second Order to continue pursuing Typhus, while the other Orders would travel to nearby Systems, to search for the Primarch Leman Russ and to attack any Traitors they came across.