Index Astartes IV: "Sons of Horus: The Black Legion''

 Unprepared and unarmoured, the Emperor and Horus fought back to back until a plasma blast stunned Horus and sent him staggering to the floor. The Emperor stood over the Primarch and refused to give ground until reinforcements arrived to drive their attackers back. On the Ork-infested planet of Gorro, Horus repaid the debt by hacking the arm from a huge, frenzied Greenskin warlord as it struggled to choke the Emperor's life out of him.