Arcane Angels
Monte Cook's Book of Eldritch Might II includes Arcane Angels , celestials which have left the celestial chorus and become agents/devotees of magic itself. They'd be great servitors of Istus or Boccob. Monte described 2 versions, I think one was NG and one was N, but you could easily make them both N agents of fate.
You could make some sort of Norns/Grey Ladies/ Furies/ Fates kind of agent who always shows up as a group of 3 and they are never permanently destroyed. They could have a nice name like "The Kindly Ones" (see Sandman comic for more ideas) and a not-nice name like "The Hounds of Istus". I would think that the agents of Fate would be implacable--that would define them the way glorious beauty defines the Celestials.
Tailor a celestial but put a different face and spin on it; that'd keep your players guessing.