Gravity throne hologram
From Halopedia, the Halo wiki
The gravity throne hologram glitch is a load-related glitch present in the Halo 2 Campaign level Delta Halo. The glitch causes the Prophet of Regret's hologram to vanish from the Holo-Pedestal that Cortana translates for the player, and it also prevents some Honor Guard Elites from appearing; however, the gravity throne shown in the hologram is still present.
One method is to use a Ghost to perform The Delta Halo Vacation, allowing a player to bypass a loading point and preventing some Honor Guard Elites and Regret's image from loading. However, the gravity throne is still present in the hologram. The Floating Switch method can also be used; when a player reaches the top, they should go near the center, and jump down onto a "weird structure".
It is possible that Regret counts as an enemy, but his throne counts as scenery or a vehicle, which is why it remains when his model is never loaded. The other hologram near the ruins is also affected by the glitch.