Undertow

The clever part is the direction the rules text reaches. The obvious way to answer islandwalk is to strip the keyword from the attacker, but that creates a dependency chain: cards that grant landwalk as a buff would have to be untangled, and creatures' printed text would be silently overwritten. Instead, this enchantment leaves the attackers untouched and grants the defenders a new permission: block as though the ability were not there. The board state ends up identical, but the wording solves the problem from the defender's side of the combat step rather than the attacker's, sidestepping the whole tangle. That a dedicated enchantment slot existed for this, rather than a clause stapled to a broader effect, says how seriously early design treated landwalk as a threat. In a blue-heavy metagame, Lord of Atlantis turned every Merfolk into an unblockable clock, and a printed answer that did not also cost you your own board was a genuine concession. The card preserves a balance problem the game has largely since solved by printing landwalk far less often, but as a piece of rules-text engineering it remains a tidy demonstration that you can neutralize a keyword without ever touching the creature that has it.