Empty the Pits is... a card that exists. If you have a fully stocked graveyard (or a ton of mana), it can produce an army of tokens at instant speed, which is certainly a thing. However, there are much better finishers for both of those situations -
Living Death is usually a better way to take advantage of a full graveyard, while
Exsanguinate or
Torment of Hailfire is a better mana sink. As a result, I'm not entirely sure what niche Empty the Pits fills. I suppose it could do work in zombie tribal, but
Army of the Damned exists... plus many zombie decks want to keep their graveyards full due to the recursive nature of many zombies.
Hmmm.... I think it suffers a bit from the fact that delve is an extremely difficult mechanic to balance -
Treasure Cruise and
Dig Through Time ended up being pretty busted, while stuff like
Temporal Trespass went the other direction and ended up almost unplayable. There's not a lot of middle ground other than on creatures (due to the fact that even a
vanilla 1 mana 5/5 still trades evenly with any
Swords to Plowshares). Empty the Pits can have a very strong effect (given sufficient support), so it was costed pretty conservatively as a result. Would it have broken Standard at
XBBBB or
XXBB? No idea.