For those that don't know it has stealth scenario's, combat scenario's, running scenario's and puzzle scenario's. There are 10 different 'games', and most of them consist of up to 6 rounds. For example there is a map where you have to steal 6 items from a 4 floor mansion filled with guards and hidden rooms, there is running trial with powers, there is a puzzle map where you have to kill all enemies in a single slow-time, etc.
Does it add any runes or bonecharms to your character? Can you play it before you end the game?
No it doesn't add any runes/bone charms. I find it useful for getting used to the controls and different kills, and trying to stay unnoticed by guards...good practise for getting ghost and other achievements/trophies.