From Halopedia, the Halo wiki
Unseal the Hushed Casket is a video promoting the release of Halo: Combat Evolved Anniversary in Halo: The Master Chief Collection on PC. It was released on March 2, 2020.
The video features the soundtrack Yawning Chasm and dialogue from the level The Pillar of Autumn playing over the Halo: Combat Evolved Anniversary menu background animation.
The title alludes to Cortana's transmission to Sam Marcus and Thom Shephard in the opening cutscene of the level The Pillar of Autumn.
Various shots of John-117 in his Cryo chamber play.
MJOLNIR Mark V armor energy shielding can be heard recharging.