Death knight

[JackPhoenix]

A really high level Oathbreaker is overpowered. A high level Oathbreaker can literally have a CR17 Death Knight or CR18 Demilich slave.

At low levels, they're not overpowered at all. Very strong, but not too bad. An Oathbreaker + Necromancer pair are overpowered though.

It's less overpowered than it seems. First, he would need to find the undead he wants (and if the GM is worried about that, he'll simply never include one) while being of sufficient level. Then, he'll need to make the undead fail the save... Death Knight has +9 on wisdom saves, good luck with that. The need to renew the control every 24 hours is a minor obstacle.

And we're not talking about zombies: even if you do get command of such powerful undead, they are intelligent, and while they do have to obey your commands, they may interpret them as they see fit. Just because you have control over them doesn't mean they like you. They propably hate you even more than usualy, and do anything to subvert your orders.

A death knight is a CR 17 undead in the Monster Manual.

Hopefully, this is not what your player meant. But I thought it worth pointing out.

When players says something like "I want to play a death knight", I automatically take it that he played WoW, and wants to play WoW DK, not semi-obscure high level D&D monster.