If you want to use the customization that is fine, you donât have to come up with something crazy to justify having red eyes. Not all elves with red eyes have to be undead but if youâre looking for lore on dark druids, then the old RPG guide had a class called âplagueshifterâ that I liked. While the RPG guide isnât canon, it still has ideas that arenât a stretch to RP as.
"Unlike others, the wise races of the Horde realized that the plague must be fought intelligently and skillfully, through knowledge and not with wild sword strokes. After long and careful deliberation the Horde formed a new order of druids, the plagueshifters, who were charged with reclaiming the Plaguelands and other stricken areas for the Horde.
Plagueshifters fill a specialized role, one typically taken by tauren druids, who are willing to leave Kalimdor and walk in far lands. To become a plagueshifter, one must risk exposure to the deadliest diseases and poisons, and learn to master them. Itâs a dangerous profession, but just as the mastery of sharp steel has its rewards, so too does the victory over the most insidious enemies of the natural world."
There are also worgen to consider, who originated from being druids and Goldrinnâs powers. Wowhead has this to say:
âThe worgen curse makes raising them into undeath far more difficult than it is for normal humans. The worgen curse has roots in both the Emerald Dream (through the wolf Ancient Goldrinn) and the holy power of the goddess Elune. In addition, those worgen who imbibe the waters of Talâdoren â during the Ritual of Balance â have a further resistance to the corruption of undeath. Sylvanasâ Valâkyr are less powerful than the Lich King and are unable to overcome this resistance and turn worgen into Forsaken, but the powers of the Lich King himself are sufficient enough to raise worgen as death knights. Worgen death knights can shapeshift, such as Grant Lazarby. Forsaken can be cursed to appear as worgen, like Apothecary Berard.â
So I think it could be possible to be an undead druid, but not very common. If a worgen could retain their ability to shapeshift as a death knight, it may be possible for a druid to retain their abilities in undeath as well.
There are also other options for being a bit more shady/villainous as a druid too. Such as Druids of the Fang, or the Druids of Nightmare. There are so many reasons your druid might look different, aside from just being darkfallen.