The Fidelitas Lex was covered in the religious iconography of the Word Bearers and was home to many crazed Cultists and faithful followers. These pilgrims lived in squalor, giving much of the ships deep interior a filthy appearance. One exception was the Hall of Anamnesis, which held much of the Word Bearer dead and was site of a statue of Cyrene Valantion. Lorgar himself commanded the vessel from the Peregrinus Basilica.
The massive battleship participated in Lorgar's pilgrimage into the Eye of Terror as well as the Shadow Crusade. During the Shadow Crusade, the Lex finally met its end, overwhelmed by a fleet of smaller Ultramarines ships over Nuceria. Heavily damaged, the vessel crashed into the planets oceans, sending a tidal wave across its main continent. By this point the vessel no longer served as the flagship of the Word Bearers, with Lorgar having transferred himself to the Trisagion.