Extreme Demons are extremely hard Demon levels which difficulty is far beyond the difficulty of most Demon levels. Majority of these levels have been completed legitimately by a few players. These levels require god-like skills to be completed.
As of September 8, 2018, there are 90 Demon levels which are officially rated Extreme Demons by RobTop. In addition, there are also several upcoming, dead, or unrated Extreme Demons (note that these ones are not included in this category).
Levels easier than the Extreme Demons are the Insane Demons, while levels harder than them do not have a difficulty (levels harder than Extreme Demons are basically impossible levels verified using hacks or glitches).