Both branches of the protoss warrior tradition share the ability to merge their souls. Twilight archons radiate incalculable power and can unleash devastating psionic storms against enemy forces both in the air and on the ground.
The first (and only confirmed) twilight archon was formed shortly after the Fall of Aiur from a high templar named Rihod and a former high templar cut off from the Khala, Lekila. The merged form possessed considerable power, devastating zerg forces around it in a highly visible display. It could even teleport objects. A Dark Templar commander considered it a sign of the future.
The true abilities of the twilight archon are shrouded in mystery, though it is clear that they were more than capable of taking care of themselves and dishing out considerable damage to clumps of weaker ground units. They were described as "attacking with psionic storms" which is identical to the archon description in the StarCraft I manual.
Unlike the archons in original StarCraft, the psionic manifestation of the twilight archon matched the team color.