Extreme Monsters

Extreme Monsters have yet to come out in the TCG/OTG. All extreme monsters start with XE, otherwise they are fakes. They cannot be special summoned, only normal summoned by Extreme Summoning.

They made they're first appearence in the Dark Revolution booster packs (only in manga so far). They are not in the Extra Deck, just like rituals.

Extreme monsters are a good addition to a deck because they will never get banned, as they all need you to pay life points to activate their bizzare effects.

This is by-far all the information avaliable.