The death knight - and there are only twelve of these dreadful creatures known to exist - is a horrifying form of lich created by a demon prince (it is thought Demogorgon) from a fallen human paladin. The death knight itself cannot be turned or dispelled (though it is affected by holy word) but it has power over undead equivalent to that of a 6th level cleric. It has 75% magic resistance, and if 11 or lower is rolled on percentage dice a magic spell will be reflected back at the caster (Fiend Folio pg 23)
If you have a paladin that falls from grace in your campaign what an epic fall he could take! And the idea that there are twelve of these creatures out there performing acts of god alone knows is tantalizing. There are so many adventures that you could find where one of these guys could be the main villain and you could focus an entire campaign around the idea of defeating each of the twelve death knights.
This version of the Death Knight, which is uncluttered by the likes of Lord Soth and all that bullshit baggage that comes along with him, is perfect. He is inspired in his concept. The art for him is magnificent, and you could not ask for a better villain to drop into your campaign.