Art by   thecrazydragon
You can fave the artist's submission here: https://www.furaffinity.net/view/29216720/

Holy lordt I can't believe I never posted this before! Amazing amazing amazing piece I got from   thecrazydragon a few years back. I can't believe it's been that long, honestly.

Datura is a cursed undead unicorn: he is anchored to the physical world by his horn, and as long as it exists, he can regenerate his body from any damage it might take. This takes an extremely long time unless he has "materials" he can assimilate, generally any kind of organic matter, ranging from scavenged dead things to plants.
Or in simpler terms, he's sort of a plant zombie, but he hates being called that. It's also why he has to use a glammer to make himself appear more "normal," because while he can control his mass and size to some extent, he can't hide all of the telltale signs of being undead without some helpful illusions to hide the random plants that tend to grow all over him.
Don't worry, though, he's not really rotten or anything. That's more Jack's deal than his. He's also a lot more "lazy and wants to be left alone" rather than "psychotic evil."