Quickly summarized: Just about the best game I have every played in my life in basically all important aspects.

Dishonored submerges the player into a dystopian, plague-ridden steampunk scenery set in the past of a fictional version of London, UK. After the brutal murder of the legitimate monarch, corrupt forces bring the so called Royal Protecter (the player and also the former protegée of the assassined Queen) into a position in which he sees himself destined to free the political regime from said influences. This path, the player - driven by explained intentions - followingly is walking upon, repeatedly brings Corvo (the Royal Protector) in multi-faceted moral predicaments that force him to make decisions with drastic implications and consequence for the developement of the story overall in the game. One could argue, what makes this game stand out is its mind-blowingly complex combat system or just the ingenious story-telling by itself which poetically describes an alternate reality that any experienced reader would likely devour in seconds if it was only a book. But no, what in reality only this game has managed to implement so far, is a constant feeling of moral ambiguity in the player that constantly makes you think about your decisions and ethical judgements. No game has made me feel as bad for merely treating a lifeless NPC in a hostile manner.
Not later than after ones first playthrough, one is fully confronted with the consequences of the accumulated amount of all decisions that have been made during the process of playing the missions.
The feeling of moral unsatisfaction combined with the (verifiable) presumption that there is much more content to explore even after a whole playthrough, makes one make a second attempt at the game, then a third and a fourth, a fifth, (...).
And there might be many things that are unsure but what is certain, is that Dishonored 1 will never fail to surprise you with new parts of the game, new storylines or places, no matter how often you reiterate and play again.

10/10