For as many times as it's been reprinted, the enchantment Grave Pact has retained its value quite well since first being printed all the way back in Stronghold. It was most recently reprinted in the Devour for Power Commander 2011 pre-constructed deck. It's been a casual favorite for many years, and in the right Commander deck, it can devastate opponents' forces into submission.
Grave Pact has been printed in Stronghold, Eighth Edition, Ninth Edition, Tenth Edition, a Planechase deck, and the aforementioned Commander deck. It's still not a cheap card even with 3 core set reprints. That's because copies are always in demand. With mono-black decks often having many sacrifice outlets, this is a must-have.
Grave Pact reads:
Whenever a creature you control is put into a graveyard from the battlefield, each other player sacrifices a creature.
This enchantment is especially powerful in a multi-player format. It's only 4 mana, so it easily drops early in the game. Because it has three black mana symbols in its cost, it can be tricky to play outside of mono-black or a two-color deck. But for all of the mono-black cards that benefit from devotion (the number of Black mana symbols in permanents you control), getting three devotion that easily is pretty solid. This card is a great help to cards like Erebos, God of the Dead and Grey Merchant of Asphodel.
There are also countless combos that interact with Grave Pact, especially any that can create recurring sacrifice effects. Mox Diamond has a pretty exhaustive database of them. There's also a functionally similar card to Grave Pact called Dictate of Erebos. The only difference is that it costs 3BB and can be cast at instant speed. Grave Pact is less mana, though, but both work well together. If you want to go all in on the sacrifice theme, you can add Butcher of Malakir for a creature-based version that gains you an additional sacrifice if the Butcher dies, as well.
Grave Pact is a Commander staple and while Dictate of Erebos and Butcher of Malakir are cheaper, the original Pact is worth the extra investment.