The quest leader either sometimes or always asks for the quest artifact back when you return with it, to replace the weird flavor in most quests that the leader is desperate to get the artifact back but is then completely fine with you adventuring off with it after retrieving it.

If you refuse to give them the artifact back, they turn hostile. If it’s a chance-based system, the chance might be dependent on your alignment record, so a character that has misbehaved is less likely to be found worthy of keeping the artifact. However, since you must have +20 alignment to enter the quest and get a massive amount of alignment from killing the nemesis, it’s questionable whether anyone’s alignment will ever be in a bad enough state to increase the chances. Luck has also been suggested as a variable that influences the chance, but most players will already have maxed their luck by the end of the quest.

In order to keep the player from just dodging the quest leader on their way out of the quest so that they can keep the artifact, various enforcement mechanisms have been proposed:

• Incentive-based; returning the artifact gives you +5 alignment record (and set to 0 if negative) and +3 to luck. Rewards should be better than the incentive of killing the quest leader to get the artifact back, or else penalties applied if you kill them (e.g. the artifact blasts you even though it’s your own quest artifact).
• Strict: you must show the artifact to the quest leader and have the quest marked completed in order for the invocation to work. Alternatively, the quest leader has the Bell of Opening and will only surrender it when you present the artifact, whether or not they demand it back. Or the quest nemesis has cursed the Bell and the leader will lift the curse after you show them the artifact.
• Delayed: you don’t have to go without the artifact forever, but the leader demands it back for some time so that they can fix whatever problem the artifact needed to solve. This could manifest as either a variable number of turns, or a requirement to go explore a certain number of new levels/area, before they’re done with it.