Raging River

The mechanic is a pile-split: whenever you attack, the defender carves their groundbound creatures into a "left" pile and a "right" pile, then you, knowing both piles, assign each attacker to a side and can only be blocked by that pile (or by flyers). The structural elegance is that neither player gets a clean answer. The defender must commit blockers to halves before knowing the assignment, so any pile holding a key blocker leaves the rest of the board open; you, in turn, only pick a side, never the blockers themselves, so a savvy split can still tax every swing. Flying sidesteps the whole apparatus, which is the period-appropriate way of saying this breaks a ground stall rather than granting hard evasion. The design points forward to every card that forces a defender to pre-commit information before blocks are declared, and Master Warcraft would later explore the same impulse from the opposite direction by letting one player dictate the entire block. The double-red cost and the requirement that you actually be attacking mark it as a red aggressive enchantment from an era when red was still being handed tools that looked nothing like burn.