Others have already explained it, so I'll just add some history. This card:

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....resulted in what was called "Black Summer of 1996" where 8 of the top 8 of every tournament was a Necro variant. Necropotence is an incredibly well-balanced card. You can't pay a bunch of life to go over your maximum hand size and use that to discard a creature to Reanimate. You don't get the cards until your turn is over. More importantly, it was printed with this card in mind:

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At the time, Black Vise was used as a four-of in every aggro deck. So it's not like you could play Necropotence on turn one, draw back to 7 cards, and being able to survive long enough. You'd end up locking yourself out of being able to draw more cards by the time your second turn started because you'd take a bunch of damage to the face.

In the spring of 1996, Black Vise got banned in Type 2/Type 1.5/Type 1.5x and restricted in Type 1. By summer, people figured out Necropotence was absolutely busted when your life total wasn't under pressure each turn from multiple copies of Black Vise being in play. You could play a bunch of low-resource cards each turn, then pay 4~8 life to draw more cards and repeat. Non-Necro decks couldn't keep up with the card draw.

Black Summer would end in fall of 1996 not with a ban, but with the creation of a deck called "Turbo Stasis" that prevented each player from accessing all of their resources each turn. It didn't matter if Necro drew zero cards or 60, they wouldn't be able to play any of them.

Since then, Magic's designers have been trying to make fixed versions of Necropotence. One of the earliest attempts to do this was Yawgmoth's Bargain. They thought maybe Necropotence had been too good because it could be played on turn 1 with the right combination of cards. So, they increased Bargain to 6 mana, instead of 3. This also turned out to be a mistake because players still figured out how to cheat it into play on turn 1. Worse, it didn't have any of Necropotence's restrictions. This meant you could perform absurd combos since you got the cards right away, one at a time, rather than at the end of your turn when you had to discard down to 7 cards.

Yawgmoth's Bargain ate a ban along with a whole bunch of other cards from the set it was released in across every format. It was restricted to a single copy in Type 1 (now known as Vintage) for almost two decades until 2017 and remains banned in every other format.

Thank you for coming to my MtG history lecture. Tomorrow I will be discussing why you should play Four Horsemen together with Eggs in your Battle of Wits deck.