Yes, I agree that they are strong. I initially suggested that knights should not have spells, and that the unholy should be strong. I'll think about it some more.

Added 7 minutes later
Unit------Dark Knight------Death Knight------Champion of Darkness---
Ability-------------------------Cursed Strike-----------Zombie Strike---

Unit----------Undead----------Death Herald----------Banshee---
Ability-------Aura of Terror----------Aura of Terror----------------Aura of Terror---
Ability--------------------------Devastating Grasp--------Piercing Wail---
Ability-----------------------------Fear Attack-------------Grief Strike---

Now it's not as overpowered, but we might have gone a bit too far. To maintain balance, we need to look back at what was there before and what other races have at levels 6 and 7. Here's my version, which I think is the most logical. Previously, the Undead had nothing at level 6, and only gained one ability upon upgrading. Here, they clearly have more, and they will also be able to teleport and fly. By the way, the knights could be made immune to slowing effects if they seem too weak now