From Halopedia, the Halo wiki
Cause of death:
Killed in action by Flood forces
Animus-To-Remain was a Forerunner Warrior-Servant of the Promethean order. They fought in the height of the Forerunner-Flood war and, alongside fellow warriors Malice-Begets-Demise and Habit-To-Sever, died on the shield world Trove. During the battle, the three Prometheans fought a sacrificial defensive effort to defend the shield world's damaged monitor 295 Harken Watch, allowing it to retreat and seal off the shield world interior from Flood forces.