Beautifully done -- well-written, well-paced, plausible despite my initial disbelief in the premise/pairing.  Dark MCGonagall is a compelling idea on many levels, and canon makes it easy to believe in a strong tension, sexual or otherwise, between Minerva and Lucius.

Many great lines; here's one of my favorites:  "His current approach had all the grace of a dropped brick, but at least it was no longer a ton of them."