"MUGEN" and "originality" aren't two words that come together often. You'll find about a thousand Gokus and enough Ryu to fill an entire arcade full of Street Fighter cabinets, so it's not often you find an entirely original work made with it. There's a great exception in The Black Heart, however, a surprisingly creepy 2D fighter brought out entirely by the one man team of Andrés Borghi. It's probably the best use of the aging MUGEN engine you'll probably find, and it's a pretty decent fighter on its own.
There's only six characters in the game, not including the creatively named final boss, Final. They're a pretty varied bunch for what you get, though, from a giant demon worm/snake thing to a cheerfully murderous skeleton boy, to a ghostly girl who's mostly one big tribute to The Ring. The plot, such as it is, involves a creepy alternate dimension where a Lovecraftian beast known as Final holds the heart of the realm's late ruler. Every character has an intro and outro that describes why they want the heart and the power it gives, but that's generally about it.
In terms of gameplay, The Black Heart feels the most like the later Darkstalkers games, only with four buttons to do your fighing with. If you're familiar with Vampire Savior, a lot of that will carry over, from the flashy combo system to the special "Killer Mode" every character can use for a temporary buff. It's a pretty easy game to learn, though, especially since none of the specials require anything much tougher than a dragon punch motion. The game does a good job of making every character feel distinct, however, with every character having some niche they're good at covering, like projectile spams or counters.
There are also "Fatal Moves" you can pull off, which work similarly to Guilty Gear's Instant Kills. These cost your entire meter and require your opponent to be nearly KO'd, but if you can pull them off, they'll instantly win you the round. You'll probably want to use these against the AI when you can, especially if you're playing on the game's harder difficulty. You'll need to play on the harder difficulty if you want any chance of unlocking Final, who's a pretty cheap jerk who has laser vision and loves to abuse it on you. Thankfully, however, you've got unlimited continues.
The general presentation's fantastic, even if the sprites aren't quite up to the standards of your average Capcom game. Aside from a few silly moments, however, the game is played for pure horror, with a lot of creepy backgrounds and atmospheric music. Sadly, there's no online play, which means that the replay value of a game like this depends on how many friends you have around. For a fighting game made a single guy, however, it's a fantastic effort, especially considering the sorts of things that usually get made with MUGEN.