のたうつ分解者

To determine the total cost of a spell, start with the mana cost or alternative cost you’re paying, add any cost increases, then apply any cost reductions (such as that of Writhing Necromass). The mana value of the spell is determined by only its mana cost, no matter what the total cost to cast that spell was.

Writhing Necromass’s first ability can’t reduce the {B} in its cost.