Basically the strongest being of their respective Celestial Object. They are ludicrously strong, are connected to their Celestial Object and are usually one in the same and tend to have some ridiculous ace(Type-Moon/Crimson Moon Brunsterd has a weapon that basically resets Humanity and Type-ORT can absorb the abilities of those it kills).
For a final fuck you, they don't have the same level of "Death" that Humans have and thus still persists even if their bodies are destroyed.