This game mechanic refers to a specific part in a game that if accomplished it will make you feel ‘epic’. Especially if it is really difficult and not many players have managed to complete that certain task. But it can actually be anything that can give you some sort of epic satisfaction like a very difficult boss fight even in a single player game.
The image above is a what the new boss from the newest raid in World of Warcraft looks like. I’m not aware if anyone has managed to defeat him yet.