Although this may look narrow, because we can only activate it when an opponent casts a Black spell, consider the following: Most pods of players have at least one person playing Black spells
As long as you can put it into your graveyard, you can Debtors' Knell it out
By presenting this card, you may gain preferential entry into the Secret Super Battle. 8
Mill is now stronger than ever, but it's still just mediocre in Commander at best, so Bruvac's deck is a few spots shy of being #1
Played early game it can also be used as effective hand disruption, and just knowing it is in a deck can keep opponents from committing to the board
Together this pair provide consistent opportunities to put a free permanent into play each turn, on top of the tutoring from Pod