The Boss World is a special level featured in both Enigmata games, and is accessible through something called "Portal to the Boss World" in both games. Two distinctive traits of Boss World are: Boss World is always a much longer level than any normal level, and all the bosses of the game will appear there save for the True Final Boss. However, it's not a boss rush, but instead all of the bosses there will be treated as normal enemies, so there'll be a lot of the same boss there, but you don't have to defeat them!
The "Portal to the Boss World" is bought for 10,000,000 Gold in the Legendary Shop in Enigmata 1, and is activated as a skill. Once that skill is activated, you'll enter Boss World. Other than that, every other information is said above. The one boss that won't appear here is the True Final Boss, Epic.
The Boss World in Enigmata: Genu's Revenge lasts exactly 313 seconds, and the Portal to the Boss World in this game can also be bought for 10,000,000 Gold in the Black Market after getting the Legendary Key. However, this time it's not a skill, but instead, when you decide to go play and a dialog appears to tell you if you want to replay a level or play the next level, a third golden button will appear that reads "Boss World". Click there to enter the level. Everything else is pretty much the same as the original game, but there'll be Enigmata: Genu's Revenge bosses instead of Enigmata bosses (obviously), and the one boss that won't appear here is instead Entity. Also, all the bosses' drones (and turrets!) here will be spawned together with their "owner", but will instead be detached from their main body and act independently! Spooky!