@ere
That's a Death Knight, but a naturally created one, not one formed from some spell or skill (undead are formed naturally from places with lots of death, like battlefields). That said, one of Ainz's weakest skills is able to create Death Knights with ease.
For reference, a Death Knight is around level 35, but has a defense levels that put it on the level of level 40. According to legends in this world, a single Death Knight is enough to destroy a small kingdom, because Death Knights are hard to kill and whoever they kill become zombies.
Gazeef, the strongest man in the Kingdom, without his epic gear, would be considered around level 30-35. (In the Web Novel, he fights a Death Knight and while he doesn't lose, he can't damage it either).
Hamsuke, a creature of legend feared by all in the Forest of Tob, enough that she held power over an area by herself while other monsters needed minions, is around level 30-35 as well.
The old man is Fluder, said to be the strongest magic caster alive, because he can cast 6th level spells, which is unheard of in the current age (where only the truly gifted and genius can even cast 5th level magic).
Even he can't control a Death Knight with his 6th level magic.
It just shows how outmatched 99% of the world is compared to Nazarick. In this world, 5 levels difference in power is enough to tip the scales in a fight. 10 levels makes it impossible to win alone.