The version we use in the DLCS (and subsequently used elsewhere, such as Lord Soth's stats in War of the Lance) differs from MM2's death knight in that the more powerful death knights gain spell-like abilities such as power words, and gain see invisibility at will. Death knights of Krynn may also create skeleton warriors (also included in the DLCS).
Tome of Horrors is still somewhat faithful to the original Fiend Folio death knight from 1st edition AD&D, but it's much more demonic in nature and their creator is given as Orcus, not Demogorgon.
Cheers,
Cam