For the ones of you who haven't played Elder Scrolls Online, Celestials are beings of immense power who, I think, normally reside in Aetherius.

A few of these beings are know under the names: The Serpent, The Mage and The Warrior.

These are the beings that give you some extra power when you active one of their Standing Stones in Tamriel (such as the ones found in Oblivion and Skyrim).

They are most likely Aedra, specifically Magna-Ge, that didn't take part in the creation of Nirn.

Because they can't truly die (as seen in Elder Scrolls Online Trials), they are probably on the same level as the most powerful Daedra, i.e. Daedric Princes.

We didn't know much about them before ESO, and even so, there isn't much to be found about them on the internet.

What do you think about these beings. Are they a kind of Aedra, Magna-Ge or something else entirely? How powerful are they?,...

The theory above is almost entirely my own, cause there isn't much know about them. That's why I'd like to discuss this subject.