Since it hasn't been mentioned yet, and I still find it enjoyable, Neverwinter Nights 2 (not he MMO) can allow for a death knight style character. Being D&D, there are plenty of options to make some very interesting characters. This is, of course, assuming that you don't mind playing an older game, and/or don't mind playing a game using the D&D 3.5 ruleset.
From an action RPG standpoint, Titan Quest allowed for a Warfare/Spirit (Spellbreaker) or Defense/Spirit (Spellbinder), both of which could fit the death knight themes quite well (life steal, debuff, summons, etc). It's too early to tell, but Grim Dawn (still in alpha) may allow for a death knight style character, but it would be using more Occult-like abilities (so similar, but different). Path of Exile could allow for such a build as well, though I'm not sure how well it would play in the late game, as I've never tried building a death knight style character. You can get life steal, summons, and some curses, probably use a Templar, go with armor/energy shield, and maybe grab Queen's Decree (unique 2H sword meant for summoners).
Rift, EQ2 and AoC are all good games, from different point of views. Rift is most similar to WoW:BC, and has some solid group content. EQ2 is huge and packed with content, but can be difficult to get going as a new player. AoC has different combat mechanics (especially for melee), and is the least polished, but Funcom makes the best dungeons in the industry, IMO.