User blog comment:The Milkman/The Writing of the Ending/@comment-24174486-20120726200543/@comment-24174486-20120823180410

The definition of "deus ex machina" from the Merriam-Webster dictionary on my netbook: "a person or thing (as in fiction or drama) that appears or is introduced suddenly and unexpectedly and provides a contrived solution to an apparently insoluble difficulty".

The key word there is "suddenly". Shepard learns about the Crucible in the third game, but it does not suddenly provide a solution. It is not something that suddenly allows the galaxy to defeat the Reapers. The characters in the game, and thus the player, are required to work for it. The more work the characters/player put into it, the better the Crucible's activation goes.

The Catalyst does not suddenly provide a solution either, because you are told long before the end of the game that the Catalyst is required to use the Crucible. While you do not learn its exact nature until the end of the game, its purpose is exactly what you are told by Vendetta. Thus it is foreshadowed, thus it is not a deus ex machina.