Part of a Win Condition or a “Win More” Card?
Maga, Traitor to Mortals may not be the most popular of mono-Black Commanders. But he is a popular and powerful Legendary creature who already has access to powerful mana doublers such as Extraplanar Lens and Nirkana Revenant. Empty the Pits is a good backup plan for Maga. He is usually intent on casting a huge Exsanguinate (which affects all players) or Profane Command (Hits only one player but provides a second effect as well). Maga can even kill a player in one shot with his own life loss ability. Being able to cast a huge zombie army to hold the line and potentially finish off stragglers is a fine plan B.
Sidisi, Brood Tyrant is the most played Commander that has co-opted the services of this Zombie army building delve spell. Already a master of her own Zombie horde, Sidisi clearly subscribes to the plan of never being able to have enough zombies. Empty the Pits feels like a bit of a win more card, but if Sidisi ever finds herself behind on the battlefield, this at the very least can be a great equalizer.
Ghoulcaller Gisa is a mono Black Commander who has steadily gained in popularity over the years. She's yet another Commander with a Zombie horde at her back. In her case, though, she's most interested in sacrificing bigger creatures to net herself more zombies. Again, empty the pits seems like a win more card. But for builds around Gisa that are purely on the overwhelm your opponent with zombies and zombie lords, this is a spell that can serve your army well with much needed reinforcements for the end game.
Tasigur, the Golden Fang is another moderately popular commander that has fooled around with Emptying the Pits on occasion. Tasigur is most interested in getting multiple uses out of his value spells and creatures. The best use for empty the pits for him is almost purely for the delve aspect. With Tasigur’s card recursion ability being random, filtering out irrelevant spells is welcome. Empty the Pits makes it so they can be exiled to fuel the summoning of a few Zombie friends. While it's not a perfect fit in his strategy, it's worth consideration.
Finding the Best Fit
Being Mono Black commanders, Maga and Gisa have access to Cabal Coffers and Nykthos, Shrine to Nyx. This makes paying for Empty the Pits a lot easier and can make the sheer number of tokens game ending. The 4 black mana symbols make this instant trickier to cast outside of mono Black. These two commanders are probably the easiest fit as far as easiness to cast.
Empty the Pits is often paired with cards such as Endless Ranks of the Dead and Unbreathing Horde, both of which greatly benefit from having a mass of zombies on the board. Even better is that empty the pits being at instant speed means that when you untap, if you have Endless Ranks on board, you get 50 percent more zombies. Lord of the Undead and especially Undead Warchief pump up your zombie army to lethal levels.
Ghoulcaller Gisa plays all of these aforementioned cards on a regular basis, along with the zombie spewing sorcery Army of the Damned. Therefore, this is easily the commander who has the most synergy with Empty the Pits. Other Zombie commanders such as the blue black Grimgrin, Corpse Born and the mono black Mikaeus the Unhallowed can certainly use Empty the Pits, but typically have more efficient combos to end the game.
Empty the Pits is definitely worth keeping around if you plan on playing any sort of Zombie themed deck. If not, it's still worth holding onto for the long term as a mythic rare that should always hold some casual interest.