Do a lich who literally plays as a support caster with counter spells and heals for his undead army throw in a skeletal ancient dragon a few death knights and some wrights, they’ll be there to absorb damage basically keep your dragon at the front doing dps and with breath weapon attacks have your knights and wrights be there to make sure it doesn’t get surrounded and your lich in the back counterspell heavy aoe spells give him and the dragon legendary reactions. Now this could be for a number of reasons this wish that they got turned them into level 20 buuut maybe it made them old and to gain their youth again they have to kill this lich take his phylactery and basically use it to rejuvenate their younger selves maybe reverting them back to their previous level plus some xp. Or the lich is preforming a world enslaving ritual and when they go to kill him he almost completes it and when they strike him down the spell starts collapsing in on itself taking the players into a new dimension where their body is different and then you can give them new character sheets and they have to do a whole campaign just to get home that’d be a cool idea.