Supplicants are available in Co-op Missions when playing as Alarak. They are produced in pairs, in a manner similar to zerglings. They automatically sacrifice themselves to keep Alarak alive when in close proximity to him. They can also be sacrificed to fully restore the energy of an ascendant.
While numerically superior, supplicants are very poor units offensively, dealing little damage at a short range. However, they partially make up for that with their massive health: 150 shield points and 5 armor on them (when fully upgraded) and 75 health points make them very resistant, not counting that they are warped 2 at once.
In general, it is a good idea to always have a few supplicants, because Alarak can take large amounts of damage when focused on by bigger waves, and it's always a lot worse to lose an ascendant instead of a supplicant. They also make for excellent tanks/meatshields for the rest of the army when positioned correctly.