This card's effect is a Trigger Effect. Setting the ATK equal to the number of Zombie-Type monsters x 1000 is part of the resolution of the Trigger effect.[1]
If you return face-down Zombie-Type monsters to the Deck they are not counted towards the ATK of "Kasha."[1]
If "Skill Drain" is activated, the ATK of "Kasha" becomes 0 and stays at 0 even if "Skill Drain" is later removed from the field.[1]
OCG Rulings
"Kasha" is a Special Summon monster. *This card cannot be Special Summoned, except by the method written on the card. It cannot be Special Summoned from the Graveyard, etc, with another card effect.[2]
The effect of "is the number of Zombie-Type monsters returned to the Deck by this card's effect x 1000." resolves as part of the effect which returns all monsters other than "Kasha" to the Deck. It does not start another Chain.[2]
If a non-Zombie-Type monster's Type is changed to "Zombie" by the effect of "DNA Surgery" and is returned to the Deck by the effect of "Kasha", then it is not counted as a Zombie-Type monster being returned to the Deck. The ATK of "Kasha" will be 1000x the number of monsters returned to the Deck whose Types were originally "Zombie".[3]
The ATK of "Kasha" is determined by the number of returned monsters which are Zombie-Type while they are in the Deck. If "Zoma the Spirit" is returned to the Deck, it is not a Zombie-Type monster while in the Deck, so it is not counted. Token Monsters are not returned to the Deck, so they will not be counted.[4]
When determining the ATK of "Kasha", Zombie-Type Fusion and Synchro Monsters which are returned to the Extra Deck are also counted.[5]
The ATK of "Kasha" is equal to 1000x the number of face-upZombie-Type monsters which were returned from the field to the Deck. Thus, if a face-down Zombie-Type monster is returned to the Deck, then it is not counted as a Zombie-Type monster being returned to the Deck by the effect of "Kasha".[6]
↑Konami FAQ: If monsters which is a Zombie-Type monster because of "DNA Surgery" is returned to the deck by the effect of "Kasha", then what is the ATK of "Kasha"?
↑Konami FAQ: If "Zoma the Spirit" or a Token Monster is returned to the Deck by the effect of "Kasha", then what is the ATK of "Kasha"?
↑Konami FAQ: When the effect of "Kasha" changes its ATK, are monsters returned to the Extra Deck also counted?
↑Konami FAQ: If a face-down monster is returned to the Deck by the effect of "Kasha", then what is the ATK of "Kasha"?