The only thing you need to know about Dreamkiller is that it's not very good, and you should spend your money on a different game. If you want to know why, read on.
You play Alice Drake, a woman who can enter people's dreams and fight the monsters she finds within, easing their minds and restoring mental balance. She finds her patients sleep being affected by hordes of nightmare creatures, and learns that there is a malign force behind the invasion. Dreamkiller is a form of first-person shooter, very similar to the game Painkiller apparently, though I've never played it. You have two weapons you can switch between, each with an alt-fire that has quite a different effect. E.g., the shotgun has a freeze bolt, and the minigun fires a rocket. The weapons can be upgraded, and are all roughly on a par, except for your starting weapon, which is rubbish.
The graphics are adequate, being neither particularly impressive nor very bad. The graphics on the between-levels cutscenes however, are
bad, with Alice looking like she has been painted by Edvard Munch. Each of the levels is based upon one patient's fears - fear of spiders, heights, etc. There are only two or three types of creatures in each level, though each level has its own themed enemies. They may drop something on death, which can be health, concentration (which fuels your teleport or starting weapon) or a power-up, which if you get enough, upgrades your weapons.
It's gameplay that's the big letdown. It is very simple. Run around, shoot enemies (or sometimes the things they spawn from), pick up anything they drop. Keep doing this until they are all gone and you can move on to the next area, where you do it again, and again. After a number of these you face a level boss and then move onto the next level. As the levels progress the enemies get more dangerous and take more damage, so you have to spend longer doing the same thing each time. Little thought or skill is involved. It is unimaginative, uninteresting and repetitive. Some enemies exist in an alternate state, where you can't hurt them until you switch into that state by passing through a portal, and then you can't hurt those in the original state. You may also become berserk if you kill enough enemies quickly enough. Neither makes any real difference to the basic weakness of the gameplay.
There are small signs of imagination, like one battle set within a patient's heart, but these are the exception. Health and weapons are reset at the start of each level, so there are no interesting choices to make or skills to acquire. It is not even that Dreamkiller is a particularly bad game, which could be interesting in its own way. Rather, it is just a dull, mediocre one, with little reward for playing.