you're right, i tried to do it in ygopro with no success. i understand why not with king of the swamp but beastking of the swamps doesn't say "specifically listed on the fusion monster card", just any fusion material. so is it falling back on the "hero" monster requirement? because it can be any hero and not a specific monster, so it's more of a variable than a set parameter.
All fusion subs, any of them, has always been ruled to only replaced SPECIFICALLY NAMED material.
If the material is more vague (1 "Hero" monster), then Fusion Subs can't replace it.
Beastking of the Swamp and King of the Swamp can be used as AbZero's material because they are WATER, thus they fulfill the WATER requirement without needing their effect. But they cannot replace the HERO material.
Anonymous
