The Shlangii were mercenaries, and considered the most feared ones in the known universe. Their homeworld was also known as Shlangii.
To take back the crown of Levithia, Graff Vynda-K wanted to obtain jethrik so he could fund an army, which he thought could include mercenaries from Shlangii. (TV: The Ribos Operation)
A number of Shlangii were stationed on Ry'leh to guard Azathoth. They were unreceptive to new ideas, therefore considered insusceptible to her mind control. However, some were converted into her service anyway and helped her escape her prison in Kadath in the Cold Wastes. (PROSE: All-Consuming Fire)