So this was actually awesome/hilarious. They did this a while ago, but I hadn't listened to the bonus disc of Deceiver of the Gods since I got it, so I just found out about it.
These weren't actual covers. They wrote songs in the style of Motorhead, AC/DC, Judas Priest, and Sabbath.
The Motorhead and AC/DC ones are awesome/hilarious. The instrumentation on all them is really convincing, though the voice is just hilarious 60% of the time.