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What occurs if I attack with Volcanicus the turn I combined while also playing Steamroll that same turn (assume I used a Brainstorm or Multi-Mission Tool on Divebomb) and flip enough Orange pips to KO the defender with Volcanicus' ability. Do I still calculate my total ATK damage to determine what (or if) Steamroll's damage would be?

In this case, Volcanicus has KO’d the defender before actually doing attack damage to it, so the attack will end with no attack damage dealt. Thus, Steamroll’s effect would not apply
Source: Wizards of the Coast (2019-03-15)    【Link to Ruling】
Tags: Volcanicus, Fiery Champion