From Halopedia, the Halo wiki
Thom, along with Tech Officer Samuel Marcus, unsealed Master Chief John-117 from his cryo-chamber before the Battle of Installation 04. He ran the Chief through several diagnostics and tests for his MJOLNIR Mark V armor. Soon after, Captain Keyes called the Chief to the bridge of the ship as a boarding party of Covenant arrived. A single Sangheili entered the control center of Cryo-B, killing Marcus. Shephard then told the Chief to follow him to the bridge and rushed out the door, where he was killed by a conduit explosion in the wall.
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