Lost Legendary is an easy-peasy conditions. Literally you can't use Legendary card in this battle. With the scarcity of Legendary card, this ruleset create less constraint for some players; and more to it these cards usually cost some extra mana, which means they less useful for low mana cap battle.

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Twitter :

Battle Challenge : BATTLE MAGE SECRETS : LOST LEGENDARIES RULESET#SPLINTERTALK#SPLINTERLANDS#PLAY2EARN#ONEUP#LEOFINANCE#BDVOTER#NEOXIAN#STEEMMONSTERS#BATTLE#MAGEhttps://t.co/12dmwStvN7

— Kid Miniatures (@kid_miniatures) August 13, 2023