The Shamash appear as creatures of etherial beauty with hypnotic patterns of light spilling from their forms; this light gives them abilities as simple as better night vision to more spectacular powers such as easy sway over others and the ability to avoid being hit. High-Torment Shamash reflect their rage not in darkness, oddly, but rather in terrifying forms created by illusion, extra attacks, and the ability to distract foes.
The Shamash are among the Fallen that did not appear until after the Rebellion; all angels had a tie to light in their very creation, so a specialty was not needed until some decided that humanity needed help in harnessing the power of light. The Shamash did not create light (that fell into the hands of God), but they mastered it, taking its form, forming color and illusion, and working with other angels to bring light to their creations. The Shamash were masters of desception during the Rebellion, using their light to distract and decieve, but it all proved useless in the end. Cut off from light in the Abyss, the Shamash took bodily form for the first time in their existance. They were only all too happy to escape.