This game is the reason I still have my SegaCD System.
I owned the Genesis version before this and wasn't too impressed. The CD version was so much better that that. Around the time I got my hands on the CD version, Mortal Kombat was the king of the fighting games and being able to perform a fatality was considered impressive. While the graphics of EC may not have been able to compare with an arcade game, the gameplay and features of Challenge from the Dark Side blew Mortal Kombat out of the water. Each character had enough specialty moves to keep your opponent guessing what you were going to do next. When your opponent was the computer, it cheated like crazy. There were periods where the computer would just block everything you would throw at it. And the Eternal Champion had a pattern to beat, but it wasn't a pure pattern and required great reflexes. Then after you beat him, the dark Eternal Champion usually came up and whooped me good. to this day, I still have not beat this game.
Besides regular special moves, the each character had it's own fatality. Each arena also had it's own fatality, sort of like the bridge arena in Mortal Kombat where an uppercut threw your opponent onto the spikes below, only every arena had it.
The characters were interesting. One of them is Blade, the now famous Half Vampire played by Wesley Snipes. The rest are less memorable, but still fun to play. Vampires, Pirates, Acrobats, Assasins and my leats favorite, the Caveman. Even though his arena kill was cool with a dinosaur coming on screen and noisily chewing on the loser if he fell on the left side of the screen.
Sure this game is old, but if you judge it against other stuff from it's time it is great. It's kept my interest for 10 years now. That's why I give it a 10.