Sangheili Point Defense Gauntlet
From Halopedia, the Halo wiki
An Arm Shield is a protective device carried by the Sangheili of the Covenant and later, the Separatists.
This shield was later replaced by the Elite Personal Energy Shield. While the Elite Arm Shield was never seen in game, it was used by Rtas 'Vadum in the Halo Graphic Novel.
Theory
There is a rumor that it was swapped for the full body shields the Sangheili had prior to the Battle of Reach and Battle of Installation 04. To destroy the shield, you may have needed to wear down each of the 3 layers of the shield to get a clear shot and after some time it would regenerate.
Trivia
- This item was going to be used in Halo: Combat Evolved by Sangheili soldiers, though was replaced by the Personal Energy Shields.