User blog comment:The Milkman/The Writing of the Ending/@comment-24174486-20120726200543/@comment-24174486-20120727141854

" 'a supernatural being, who is worshipped as the controller of some part of the universe or some aspect of life in the world or is the personification of some force.'- From Dictionary.Com."

The Catalyst is not a supernatural being and it is not worshipped.

"It's contrived, sudden, and unsatisfying."

You're told well in advance of the existence of the Catalyst on the Citadel by the Prothean VI, Vendetta. It even hints at the existence of a creator for the Reapers.

Calling the Catalyst a deus ex machina is inaccurate because without the Crucible, the MacGuffin you've been building for the entirety of the game, the Catalyst would be just as powerless. The Catalyst doesn't make the options available for you; it tells you what options are available to you. The mere fact that the Catalyst being on the Citadel is foreshadowed means it is not a deus ex machina.