In the kingdom of Cavalier, the King has married a young, beautiful (and buxom) young woman named Fatima. Since then, the kingdom experienced peace and prosperity, conquering neighboring lands to expand the empire. Fatima was seen as the harbinger of such blessings, therefore many worshiped her an awful lot, as if she was goddess.
So it was no surprise that the House of Blood (a family of knights who served the king) were killed for accusing Fatima of manipulating the the king. Three members of the family: Remus, his bride-to-be Sylvia (and the king's daughter!) and his brother Romulus, were incredibly upset for being unfairly treated. So they decided to make a deal with this unknown entity known as "The Beast" to be revived, as long as the three will serve every whim of this enigmatic force.
So now, the three knights storm into Cavalier, crushing any resistance whatsoever in order for their quest for revenge...
Undead Knights has similar qualities to the hack-and-slash Dynasty Warriors series: You control one character, sending him/her into hundreds of foes (and some bosses) to slice them apart. However, the one major difference is the zombies. Your character is able to convert normal soldiers into zombies, and you could order them to clear obstacles, take down structures and/or enemies. In addition, when converting soldiers, you will fill up a gauge of the ultimate attack, and to refill your health bar.
Each character plays nearly the same and, besides the zombie-converting ability, they could jump, roll-away, and slash with their normal weapons. They also could use an ultimate attack, which covers a lot of ground and could convert the neighboring dead soldiers into zombies.
Each of the 20 stages has a very similar format: You have a bunch of soldiers to kill and convert, take down some buildings and some bigger boss characters (like the large knights known as Dreadnoughts). At the end of the stage, you'll be graded in how many zombies you convert and how fast you finish the stage (they use letters, like A and B) and this could help you purchase upgrades, like increased health, a charge attack etc. Each stage also has up to 3 difficulty levels (Easy, Normal, Hell).
Undead Knights also offers a multiplayer game, although it isn't a co-op campaign.