As with the other ways to get dungeon treasures (other than, of course, actually finding them in a dungeon), this should be removed. It's called from out of SHANDALAR_encounter_enemy().

By request of Bog Wraith, though, the removal should be configurable; this is acceptable, since you have to get the card out of a dungeon at least once first. (As opposed to randomly finding something like Timetwister when you step onto your first lair.)

Will have to be careful to disallow the case where you get this reward and you own nothing except dungeon treasures, probably by not giving the reward at all.

I see there's also code and localization strings for a "You get any card of your choice" duel reward, and a Wise Man thingy for "To assist you I shall fashion 3 [jewelnames] into a duplicate of any spell you possess", but I haven't ever seen them. Maybe they're difficulty-based, like the start-of-duel option to answer a quiz to avoid fighting?