In the Garden of Ghosts - S1.E6

A small party of Aeldari return to the ruins of their ancestral craftworld to seek the spirit stones of the dead, so that those lost in battle might be returned to a semblance of life, and their memories preserved. The Warlock Iocarus has a more specific agenda, one which can only be fulfilled by reliving the agonising events of the day the craftworld fell.