Nemiel was formerly a knight of the Order, a veteran of the famous Siege of Firebase Endriago, and later elevated to become a member of the Dark Angels. Nemiel had a constant rivalry with his cousin Zahariel lasting from his youth until their separation after the Dark Angels' actions on Sarosh. While occasionally jealous of Zahariel's exploits the two remained very close through all their trials. Only one topic was taboo between Zahariel and Nemiel and that was Nemiel's participation in the conspiracy to assassinate the Emperor. Nemiel had led Zahariel to a secret meeting shortly before the Emperor arrived on Caliban. This meeting was comprised of a few of high ranking members of the recently disbanded knightly orders of Caliban. Some suggested staging an atrocity but it was eventually decided to attempt to assassinate the Emperor. Upon hearing this Zahariel left in disgust and Nemiel quickly followed him. Nemiel promised to dissuade the men of this course of action. Whether Nemiel actually tried is unknown but the attempt on the Emperor was made nonetheless. Unlike his cousin, Nemiel was allowed to stay with the Dark Angels and Lion El'Jonson while Luther, Zahariel, and the others were forced to return to Caliban.
Later in the Heresy, as the Dark Angels were battling Daemons, The Lion wished to reinstitute Librarians to combat them. This was against the edicts of the Council of Nikea, which Nemiel stubbornly reminded his Primarch. In the end, Lion El'Jonson killed Nemiel due to his refusal to back down.