Nathaniel Garro was originally the battle-captain of the Death Guard Legion's 7th Great Company and later became the commander or Agentia Primus of a band of Space Marines gathered from all the Legions known as the Knights-Errant who remained loyal to the Emperor of Mankind and were directed on missions chosen by Malcador the Sigillite, the Regent of Terra.

Kyril Sindermann would later claim that Garro became the first true martyr of the Church of the God-Emperor.