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Slifer the Sky Dragon is one of the Egyptian God Cards, the others being Obelisk the Tormentor and the Winged Dragon of Ra. Multiple versions of these cards exist, including non-tournament legal versions like this that don't have the effect printed on the card and have uniquely colored textures and backings to match. These "illegal" versions first arrived in America individually as Prismatic Secret Rares: Ra was made available with the Dawn of Destiny Xbox game, Obelisk with a Shonen Jump subscription, and Slifer with the Yu-Gi-Oh! movie ani-manga, but they eventually all came bundled together in the Legendary Collection (the version of Slifer you posted) and again in Yugi's Legendary Decks as Ultra Rares. The Prismatic Secret Rares are more desirable. (I used to have a prismatic Ra but traded it for an Asian-English Game Boy promo version because I liked the Japanese text at the bottom, but it turned out to be fake. T_T)
While the effects aren't printed on these cards, before they got tournament legal versions, many players still used them in casual duels using the anime, manga, and video games for reference. Their effects were tweaked when they finally got legal prints (some more extremely than others).
Slifer's illegal effect (from the Yu-Gi-Oh! wiki):
Spoiler:
"This card cannot be Set. This card can only be Tribute Summoned by Tributing 3 monsters. The Normal Summon of this card cannot be negated. When this card is Normal Summoned, Spells, Traps, and Effect Monster effects cannot be activated. While this card is on the field, control of this card cannot switch and the card is unaffected by the effects of Trap Cards and Effect Monsters other than "Obelisk the Tormentor" and "The Winged Dragon of Ra". The effect of "Obelisk the Tormentor" only affects this card for one turn. Your opponent cannot tribute this card. The effects of Spell Cards only affect this card for one turn. This card cannot be removed from the field by the effects of Spell Cards. This card's ATK and DEF are equal to 1000x the number of cards you have in your hand. When a monster is summoned to your opponent's side of the field in Attack or Defense Position, decrease its respective ATK or DEF by 2000. If this reduces that card's ATK or DEF to 0, destroy that card."
Slifer's legal effect:
Spoiler:
"Requires 3 Tributes to Normal Summon (cannot be Normal Set). This card's Normal Summon cannot be negated. When Normal Summoned, cards and effects cannot be activated. Once per turn, during the End Phase, if this card was Special Summoned: Send it to the GY. Gains 1000 ATK/DEF for each card in your hand. If a monster(s) is Normal or Special Summoned to your opponent's field in Attack Position: That monster(s) loses 2000 ATK, then if its ATK has been reduced to 0 as a result, destroy it."