Ascendants climb the Chain of Ascension by killing, directly or otherwise, their superiors in Rak'Shir duels. Ascendants are only second to the highlord in the Chain. The lower an ascendant's title, the higher their rank (e.g. First Ascendant is of higher rank than Second Ascendant). After years of absorbing terrazine and having the advantage of using their supplicants' essence, they become very powerful.
Ascendants appear in Legacy of the Void as a variant of the high templar. They are the Tal'darim faction option in the "psionic warrior" category, alongside the high templar and the dark archon. The ascendant's offensive abilities are Psionic Orb and Mind Blast, which do heavy damage to enemies, and it has the support ability Sacrifice, which allows it to drain life from an ally to restores its own energy.
The ascendant's primary advantage over the high templar is Sacrifice, which it can use to quickly restore its energy. This lets the ascendant be ready for combat right after warping in without needing to wait to store up energy. This also makes the ascendant powerful for prolonged conflicts, able to keep its energy reserves high to use its abilities liberally. Otherwise, Psionic Orb's potency versus Psionic Storm is circumstantial, and Mind Blast is a powerful single-target ability, both of which make the ascendant more offensive than the high templar, while the latter has Plasma Surge to support allies in combat.
If the player uses ascendants, the Spear of Adun's Reconstruction Beam ability is useful, as the ascendant can use Sacrifice on mechanical units and then Reconstruction Beam will heal them. Otherwise, sentinels and avengers are good targets for Sacrifice, since they can use their abilities to rejuvenate.