Risk of Rain is a 2D action platformer with roguelike elements where permanent death is a primary feature, Your goal is to survive as long as possible on a mysterious planet, with randomly spawning enemies and bosses.
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Pros
Pro 10 unique characters 
Imagine a small set of DOTA or LOL. These 10 characters can be tankers, DPS, supporters, rangers, carriers, etc depending on your skill and level of use. And they're hard to unlock which makes the challenging to get them even more.
Pro Replayability is high
The levels are generated very randomly making for a unique experience every time the game is played which in turn makes for a ton of replayability.
Pro Time equals difficulty
It gets tougher as time passes by. That means by minute 40, you are playing at hardcore difficulty. This makes for an experience that is good for learning the mechanics as once you can not advance due to the difficulty you will die which means the game has to be repeated which help learn the ins and outs more and more each time it is done.
Pro Fun local and online co-op modes
No need to compete as the multiplayer is all cooperative where each player has to help the other survive the harsh world which is a fun setting for friends as it leaves out any competitiveness.
Cons
Con Unlocks unclear
The game explanation of how unlocks unlock is not very clear or intuitive making for wasted time trying to figure them out.