Operation: GREY VEIL was an Office of Naval Intelligence mission to assassinate a CovenantProphet on the planet Heian sometime in 2541. The mission was carried out by a squad of ODSTs and Cal-141, a Spartan-II. The assassination mission was successful, though Cal-141 was killed in action.
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These events are depicted in episode The Babysitter in the series Halo: Legends.