March of Wretched Sorrow
Instant
As an additional cost to cast this spell, you may exile any number of black cards from your hand. This spell costs 
less to cast for each card exiled this way.
March of Wretched Sorrow deals X damage to target creature or planeswalker and you gain X life.
Tyler Jacobson
Rulings
The mana value of March of Wretched Sorrow while it's on the stack is the value chosen for X plus 1, no matter how much mana you actually paid to cast it.
(wotc • 2022-02-18)
You may exile more cards than necessary for March of Wretched Sorrow's first ability, but you can't reduce the mana it costs to less than {B} this way.
(wotc • 2022-02-18)
Rulings
The mana value of March of Wretched Sorrow while it's on the stack is the value chosen for X plus 1, no matter how much mana you actually paid to cast it.
(wotc • 2022-02-18)
You may exile more cards than necessary for March of Wretched Sorrow's first ability, but you can't reduce the mana it costs to less than {B} this way.
(wotc • 2022-02-18)