Bone Daddy vs Gramps. Out of the two beings who reign over life and death, who would come out on top?
Round 1 - Random encounter, no prep for Ainz, morals on, in character, no outside help, Ainz only has the gear he usually has on him.
Round 2 - Both have limited knowledge on each other and opt to take the other out at the same time. Ainz has as much prep as he can get during the time King Hassan is hunting him down. There's no specific time for when combat takes place and the fight starts as soon as one of them makes the first move.
Round 3 - Ainz has no idea about who King Hassan is. Hassan is convinced to come down from the mountains and assassinate Ainz. Can he break into Nazarick and manage it? Assume all defences and NPCs are at Nazarick, but it's just a regular day and they aren't on full alert.
Admittedly inspired by this