A basic summary
The Ascendants worship the wormhole aliens, which they apparently know as "the True". The True dwell in a hidden fortress and watch with eyes of fire (implicitly, referring to the wormhole and the orbs). The Ascendants are supposedly on a quest to find the fortress and unite with their gods, whereupon they'll be judged.
In their link to the wormhole aliens they're like the Bajorans and the Eav'oq (a pacifist race introduced also in the DS9 relaunch books), but unlike those two cultures they're violent zealots; long ago they cleansed whole worlds of blasphemy against the True. The surviving Eav'oq were hidden in subspace, to protect them from Ascendant purges. But the Ascendants are widely dispersed and their numbers have dwindled; there are few of them now. They rely on their advanced technology where once they possessed great numbers. They became legend in the Gamma Quadrant, but are apparently remassing as of 2376/77.
Opaka met one whose scout ship crashed on the prison moon where we left her in DS9 season 1. This Ascendant, named Raiq, disabled the satelite network and the nanotech, freeing Opaka and the trapped tribesmen and letting them survive off of the moon.
Odo started investigating the Ascendants, because they're apparently a threat to the Dominion.
The Ascendants blew up a star with a subspace weapon, killing the Progenitor Changeling and causing the Great Link to dissolve itself in shame at their failure. Apparently, the Ascendants also attacked Dominion space in some other capacity.
While Kira Nerys, now "the Hand of the Prophets", was unconscious in the infirmiry, she experienced an extended vision in which she and the rest of the Bajorans (and the DS9 crew) defended the Eav'oq and the fortress from an overwhelming Ascendant assault. More foreshadowing for a coming conflict.
Following the Mirror Universe crisis, Iliana Ghemor was deposited by the wormhole aliens at an Ascendant conclave, declaring herself "the Fire". Kira is "the Hand", as mentioned, Mirror Universe Iliana Ghemor is "the Voice". The Ascendants are massing still.
By 2381, the conflict has apparently passed - Raiq is studying under Vedek Kira on Bajor.