Advanced format ruling
Beastking of the Swamps
Card effect (current official text)You can substitute this card for any 1 Fusion Material Monster. When you do this, the other Fusion Material Monster(s) must be the correct one(s).
Current rulings for this card are maintained by Konami.
Rulings data courtesy of Format Library — formatlibrary.com. Spotted an inaccuracy? Flag it in the Advanced forum — sourced change requests get reviewed.