Lights of Sanghelios

Honor Guard Ultra is a Covenant Sangheili (Elite) rank. This rank commands the 'lower' Honor Guard Elites of the High Prophets. This rank is still equal to the rank of Brute Honor Guard since the Brute Honor Guard only has one rank within it.

They are likely more skilled than their subordinates, with many years of experience in the Covenant Honor Guard, which is particularly impressive considering the prodigious combat abilities of any in the Honor Guard.

Description
These Honor Guard Elites sport the same ornate armor as the rest of the Honor Guard, with the exception that the under armor they wear is white instead of crimson. This color is consistent with the Ultra Elites of the regular Covenant army, hence their title.

On the level Gravemind, many Ultra Honor Guards are seen without their helmet, as seen on the cutscenes

Combat
Honor Guard Ultras have never been directly seen in combat. Many players believe that one appears at the end of the level Gravemind but this Elite is actually the glitch known as the Honor Guard Councilor.

Five Honor Guard Ultras are seen in the cinematic of Sacred Icon when they, along with regular Honor Guard Elites, are stripped of their position, giving it up for the new Honor Guard Brutes. One is seen giving his helmet to another Brute.

In addition, three Honor Guard Ultras are seen killed fighting the Brutes around the middle of the level Gravemind. They wear Honor Guard armor but without helmets, as they no longer held that position when they were killed.

It is theorized that Honor Guard Ultras are more likely to withstand the Brute onslaught than regular Honor Guards, due to their superior shielding and many years of combat experience needed to become an Honor Guard Ultra.

Trivia

 * There are corpses of them scattered throughout the level Gravemind with only Energy Swords and they are seen in some of the Halo 2 cutscenes, but you do not seem to combat against a Honor Guard Ultra at all in Halo 2, with the exception of the Honor Guard Councilor glitch at the end of the level Gravemind which, of course, should not be considered a real Honor Guard Ultra.