This chapter of the guide to Dragon Ball Z: Kakarot has a walkthrough for King of the Demon Realm? side quest.
Time frame: From Majin Buu Reborn - Episode 8: Finale to No time limit
Reward: 289,729 EXP, 3x D Medal, Training Medal, Fancy Ring
Where to start: East Ravine Area, as Goku.
Your objective is to fight Dabura. The opponent should be a few levels below yours - he won't be difficult to defeat.