You will have to play through different levels filled with enemies (mainly skeletons, zombies and other 'undead') and finishing them all off. Luckily you will have a good range of attacks to defend yourself, although using them has a very high price.
The interesting turn in the gameplay of Dead of Knight is that your life bar and energy bar are the same. And to use any attack that is not basic, you will need to use this energy. So you'll have to be very careful when using special attacks.
Luckily, one of the attacks of the game allows you to grab any enemy and drain his vital energy to replenish yours.
Dead of Knight is an independent and free action game that has an excellent visual part (with support for resolutions up to 1920x1080), it's made to play with an Xbox 360 controller and it offers a few hours of good fun.