It's pretty interesting that Pazuul, despite being an Archon, doesn't appear the have a shard the way the other three do. Does that mean he's incapable of regeneration/reanimation the way the others are? Or does he not need one, because he's attached to a Sentient body, namely Erra's, rather than a Warframes? At the moment, I'm headcanoning that, in addition to allowing Archons to resurrect dead Sentient drones, Archon Shards are what they use to heal their Warframe bodies, either to prevent them from rotting, or to just heal them alongside their innate Sentient Adaptation
Edit: I'm an idiot, and he does actually have one, jammed through his head. But for whatever reason, it doesn't actually show up in his character portrait during transmissions, and he only appears in a single cutscene outside of that, so I forgot about it completely.