Games

My rarest achievement is for getting 32 silenced revolver kills in Red Orchestra 2 (0.3% of players). Technically its not too difficult, but since its a shooter with a fairly low TTK, you don't normally use the sidearm much. Further the silenced revolver is a pristige varient that has to be ground out using the original revolver, and performs worse than the unsilenced, so there is no reason to get the achievement except for completionism sake.

The achievement I'm much more proud of is Crypt of the Necromancer's Vow Down, for completing a full roguelike run with Monk. The gimmick with Monk is that the game is its normal roguelike-on-a-beat, but if you touch any gold, be it an enemy's drops or dungeon loot, you die instantly 100 to 0. This means that as well as the normal difficulty of the game, you have to plan around where you kill enemies to ensure that you always leave a safe path. I'm pretty confident that this is the most difficult thing I've ever done in a game. Technically 0.7% of player have the achievement, but given that 0.3% also have the achievement for completing the impossible New Game++++++++ character, we can effectively call it as 0.4% of players.