Yu-Gi-Oh! 10 Powerful Cards So Good They Were Banned!

Starving Venom is a Lv8 fusion monster that had 2 ways of being special summoned, either by using any 2 dark pendulum monsters and a fusion spell, or tributing those cards you control. It's effect allowed it to target another monster in the graveyard, copy its name and effect until the end of that turn, and effectively 'become' that card.

This allowed it to be summoned incredibly easily with the like of pendulum "Magician" monsters and copy the effects of much stronger cards to enable multiple uses of once-off effects, such as Electrumite's summoning effect, or to inflict direct 'burn' damage. A consistent play involved "Lyrilusc - Independent Nightingale" which was a Lv1 fusion monster who inflicted 500 points of direct damage for each level it had. Getting it into the graveyard and summoning 2 Venom's and copying Lyrilusc's effect to effectively burn the opponent with 8000 points of direct damage, ending the duel on the first turn.

It quickly became one of the most powerful cards in the game due to it's copying effect, giving it the effect of cards with strict summoning conditions, which lead to Konami needing to ban it almost immediately as it became too easy to setup and abuse, effectively breaking the game.