The T'Vanti were a primitive human society encountered by the Ultramarines during the Great Crusade. When the Imperial forces reached their world, Roboute Guilliman, the primarch of the Ultramarines, allowed them to fight alongside his Space Marines as auxiliaries in a campaign to combat the orks infesting their world.
In a strange upset, the Ultramarines' main thrust missed the headquarters of the Orks' Warboss, who was located and ultimately slain by the T'Vanti warriors. They lost eighty-nine thousand men, but felt only jubilation at their success.
One of their martial aphorisms, as related to Guilliman, was that any number of casualties was acceptable, if the battle ended in victory. While interpreting this statement for his own writings, Guilliman reflected that, for all his supreme confidence in his own Legion, he would be afraid to ask for a "T'Vanti" degree of dedication from them.
Ironically, this reflection came shortly before the outbreak of the Battle of Calth, when the Ultramarines were forced to continue fighting despite horrendous casualties, and the end of the battle could only be called a victory in Pyrrhic terms.