The Eschaton is one of the buildings that train Mega Units. You might notice that this Eschaton and the other have the same name. That is because that its the same thing, the other Eschaton is actually the ejected body of the building itself, which is why the buildings body will dissapear after training an Eschaton.
The Eschathon is also required to train the chronomancer, anhilator, and battlesphere units.