In the print version, Michael says "Lava quod est sordium!" when casting off the swarm attacking Harry.  Michael actually used this phrase at several points in the series, either as a battle cry or when invoking the power of his faith.

Since I am not Latin-literate, I'll have to rely on tools from here.  Google translate takes this and returns, literally, "Wash the dirt".  Not nearly as dramatic, so I doubt that's the intended meaning.

In the context of the scene, this can probably be more liberally interpreted as "Cleanse this filth!", which sounds a lot more like what a modern-day paladin might say.

@chepner has contributed in comment that this is also a line from a Latin Catholic hymn *[Veni Sancte Spiritus][1]*, which is presented in several different translations, but seems to be commonly used as "Clean what is unclean," and given Michael's background something very likely for him to use.

  [1]: https://en.wikipedia.org/wiki/Veni_Sancte_Spiritus