The ATK and DEF of all "Yubel" monsters are 0 and all are Fiend-Type monsters. The very last of the "Yubel" monsters can apply its effect whenever it battles, and it destroys the monster it battles. When a "Yubel" monster is destroyed, the next evolution can be Special Summoned and the player does not take any Battle Damage. When a face-upAttack Position "Yubel" monster is attacked by an opponent's monster, before damage calculation, they take damage equal to that monster's ATK.
The "Yubel" Deck revolves around destroying "Yubel" to activate its effect and Summon "Yubel - Terror Incarnate" to stall the field, and/or inflict damage each turn using other monsters.
The objective is to get "Yubel - The Ultimate Nightmare" out on the field because it cannot be destroyed by battle and can attack opponents monsters to destroy them without damage to you while damaging your opponent at no cost to you. Cards like "Montage Dragon" will fit to this Deck, as you can use the the high Level of "Yubel" for it's power.
The "Earthbound Immortals" also pose a serious problem to this Deck. Though they have high ATK, they cannot be attacked by "Yubel", while they are able to attack you directly.
Rock Decks that bounce "Yubel" back to the hand or Deck are a serious threat. Rocks also tend to have low ATK, making the whole point of "Yubel" moot.
"Shadow-Imprisoning Mirror" and "Skill Drain" can wreck a "Yubel" Deck by negating the effects of "Yubel" monsters. Even after a "Yubel" is destroyed by a card effect, "Royal Oppression" will prevent the Special Summon of the next "Yubel", making the whole effort pointless unless you revive the "Yubels".
"Stardust Dragon" is a big problem for "Yubel" Decks: It can negate the destruction and effect of all "Yubel" monsters. It can negate the damage effect of "Yubel - The Ultimate Nightmare" as well. You must have cards that can take care of it. The best is "Divine Wrath" because this Trap Card can destroy "Yubel" to "evolve" it or be used to destroy "Stardust Dragon" when it tries to negate a destruction effect and it won't come back from the Graveyard. "Compulsory Evacuation Device" can also be used to great effect on either "Stardust Dragon" or "Yubel - Terror Incarnate".
Another big problem is a "Lightsworn" Deck. "Judgment Dragon" can cause serious problems by paying 1000 Life Points multiple times to get rid of all 3 "Yubels" and attack for game. Side Decking some "Light-Imprisoning Mirrors" and/or "Brain Golems" and possibly a "Soul Release" will help to counteract the mighty "Judgment Dragon". Another good way to counter a "Lightsworn" Deck would be "Karma Cut", as it would banish a monster and all copies of it in the Graveyard, thus stalling the Summon of "Judgment Dragon". "Divine Wrath" can also be used to negate its effect and destroy it, and "Starlight Road", on top of negating the effect and destroying "Judgement Dragon", Special Summons "Stardust Dragon" to punish.
"Naturia Fruitfly" is a major problem for this Deck because of its low ATK and the ability take control of the 0 DEF "Yubel(s)".
"Neo Galaxy-Eyes Photon Dragon" is a very serious threat due to its effect, if "Galaxy-Eyes Photon Dragon" was used to Summon "Neo Galaxy-Eyes", all face-up card effects are cancelled. Also, since all forms of "Yubel" have no ATK, a direct attack from "Neo Galaxy Eyes" would end the Duel instantly.