Venga, citemos el Lexicanum
"The Emperor had saved Horus's life at the Siege of Reillis as they fought back to back. At another battle, Horus repaid this debt when he hacked the arm off a frenzied Ork as it tried to choke the life out of the Emperor on the planet of Gorro"
Y es de la epoca en la que Horus era el unico Primarca descubierto, de hecho el Emperador no intervino en Ullanor, precisamente era la prueba definitiva de la capacidad de Horus como comandante (se supone)
