Tulian Aquila was a Veteran Battle-Brother of the Ultramarines Legion who served during the Great Crusade and the Horus Heresy. He was a Sergeant at the Battle of Calth, stationed alongside his company in the city of Ithraca.
During the subsequent fighting, Aquila was moved by the sacrifice of one of his Battle-Brothers and came to the realisation -- even though he himself was already dead -- that he had a duty to protect the innocent and to suffer so that they did not have to.
Later, then-Captain Aquila went on to become the first Chapter Master of the newly created Doom Eagles Chapter during the Second Founding.
The Doom Eagles quickly embraced their Chapter Master's sense of fatalism and came to accept that death was inevitable.
They believed and that every mission performed on behalf of the Emperor must be pursued with both fervour and zeal in defence of the Imperium and its people before one's life was spent.
- Honour to the Dead (Audio Drama) by Gav Thorpe
- Legends of the Space Marines (Anthology), "The Returned" (Short Story) by James Swallow