During the Great Crusade, Aleph was part of an Imperial Fists force that fought in the Battle of Thrael Falls on Cestus II. At one point in the fight, Aleph was saved from being torn apart by the natives of Cestus II, the anak, by Captain Scaevolla of the Luna Wolves. Following this, Aleph and Scaevolla struck up a lasting friendship. They fought together on multiple occasions later in the Crusade, in actions including the cleansing of the Haruspex of Crore and the defence of the monastery of Satrapos from the ork hordes of the Starbiter.
Scaevolla, however, would later follow his Primarch, Horus, in his betrayal of the Imperium. Scaevolla tried to convince Aleph to follow him in rebellion, but Aleph refused. The two would later meet for the last time in the Siege of Terra in the shadow of the Eternity Gate. Once again, Aleph refused Scaevolla's offer and the two duelled, with Scaevolla killing Aleph. Scaevolla was so enraged by his friend's rejection that he ripped out Aleph's progenoid gland and swore an oath to the Gods of Chaos that he would hunt down and kill every descendant of Aleph's gene-seed.