User blog comment:The Milkman/The Writing of the Ending/@comment-24174486-20120726200543/@comment-3581090-20120823145109

By virtue of their joint existence and interconnected reliance upon each other in the story, the Crucible and the Catalyst are both deus ex machinas AND not deus ex machinas.

The Crucible is definitely the deus ex machina solution for the series. Because the progression of events leading to the discovery of its existence happens entirely outside of Shepard’s (and, therefore, the player’s) experience of events, it is delivered to Shepard--and, by extension, the story--with no organic narrative context. In short: it comes out of nowhere to magically stop the Reapers. We don’t earn the Crucible as a solution; the BioWare gods hand it to us.

However, if we get technical, it isn’t the Crucible that delivers the galaxy from evil: the Catalyst is the one who does all the work. And the Catalyst is given even less context than the Crucible, because while there are designs for the Crucible, there is no information whatsoever about what the Catalyst other than that it is supposed to exist. So, when it turns out to be a semi-sentient hologram kid, it’s certainly a twist--but because there is no “oh, now it makes sense!” payoff from the reveal, the Catalyst is simply another plot-resolving element that lacks any kind of organic narrative context (that is, it comes out of nowhere). Therefore, the Catalyst, the metaphorical Reaper god who waves his hand to make the Reapers stop, would have to be considered a deus ex machina.

But, again getting technical, the Catalyst can’t do anything without the Crucible, because 1) the Crucible alters the Catalyst’s programming to let it know it can use space magic, and B) the Crucible is what allows the Catalyst to use space magic.

So, to me, the issue becomes whether or not you think the Crucible and the Catalyst should be considered two separate entities or two distinct parts of a single entity.