"Deep Dark Conqueror" Hidden Challenge/Advancement

As the title says, I was thinking of a new hidden challenge/advancement that can be named "Deep Dark Conqueror". Essentially, this can be unlocked by defeating the warden. I know you don't intent the warden to be defeated so maybe you probably won't give it an item drop. However, those players who spent lots of time trying to defeat the warden should at least be rewarded with something so the second best thing is unlocking a hidden challenge/advancement.