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This spell costs {X} less to cast, where X is your devotion to black. (Each {B} in the mana costs of permanents you control counts toward your devotion to black.)
If your devotion to black is greater than six, Marshmist Titan will cost {B} to cast. The colored mana requirement isn’t reduced.
Your devotion to black is calculated when you determine Marshmist Titan’s total cost, and that cost is locked in before any costs are paid. For example, if you control a creature with black mana symbols in its mana cost that can be sacrificed for mana, those mana symbols will count toward your devotion to black. You can then sacrifice that creature for mana to pay the reduced total cost.
Hybrid mana symbols, monocolored hybrid mana symbols, and Phyrexian mana symbols do count toward your devotion to their color(s).
Mana symbols in the text boxes of permanents you control don’t count toward your devotion to any color.
Numeric mana symbols ({0}, {1}, and so on) in mana costs of permanents you control don’t count toward your devotion to any color.
