Never, ever seen the Questing Knights. It's possible it's a very low-probability event; or maybe it requires some scale combination I rarely use.
If memory serves, a search of the strings table in the executable suggests that there is at least the -idea- of giving a bonus to fire magic (Inferno) or water (Cocytus), just like coming back from the dead might improve Death magic. Can't recall an explicit mention of experience 'tho. Just because the text is there doesn't mean it's necessarily enabled... if you want to test this, try a map file where somebody has the Infernal Sword and a really bad 'tude.
And the last, heh. Double-edged event, just like the witches.
