Cannot be Normal Summoned or Set. Must be Special Summoned by "Yubel", and cannot be Special Summoned by other ways. This card cannot be destroyed by battle. You take no Battle Damage from battles involving this card. Before damage calculation, when this face-up Attack Position card is attacked by an opponent's monster: Inflict damage to your opponent equal to that monster's ATK. During your End Phase: Destroy all other monsters on the field. When this face-up card leaves the field: You can Special Summon 1 "Yubel - The Ultimate Nightmare" from your hand, Deck or Graveyard.
Special summons Yubel - The Ultimate Nightmare when he leaves the field.
Also destroys your other monsters.
Any Method of removing this monster from the field will result in summoning Yubel - The Ultimate Nightmare. You can even use him as tribute fodder.
This Yubel is the best one for board wiping so you might not want to upgrade.
Yubel has 3 forms each stronger than the last and needing the previous to be summoned. The best way to special summon the original Yubel is with Limit Reverse so that you can easily destroy it by switching battle position turning it into Yubel - Terror Incarnate which needs no cost to stay on the field. Yubel - Terror Incarnate destroys every monster on the field during your End Phase. If for any reason Yubel - Terror Incarnate leaves the field you can then special summon Yubel - The Ultimate Nightmare which destroys any monster it battles and inflicts damage to your opponent equal to that monster's attack.
The only weakness of this monster is getting put face-down. It's special summoning effect will not trigger if it is removed from the field while face-down, so by all means keep him face-up when your opponent activates cards like Paleozoic Canadia or Floodgate Trap Hole.
Since the card text has both "when" and "you can" in the text,it's an optional effect. The last thing that happened has to be the destruction of Yubel or the effect doesn't have a chance to activate.
http://duellinks.gamea.co/c/gbn3jl0a?sort=&tpage=1 for more info.
How to beat this card: Notice it says "When this card leaves the field" You just have to destroy or banish it by a card effect on chain link 2 or higher and it will miss the timing to summon ultimate nightmare, for example chain offerings to the doomed to this card's end phase effect. Also chalice makes this card an very vulnerable target, but it's effect summoning effect won't be negated in the graveyard with chalice.