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
Peakd : https://peakd.com/hive-13323/@kid.miniatures/battle-mage-secrets--lost-legendaries
X :

BATTLE MAGE SECRETS : LOST LEGENDARIES#SPLINTERLANDS#SPLINTERTALK#PLAY2EARN#LEOFINANCE#BDVOTER#NEOXIAN#ONEUP#LOST#LEGENDARIEShttps://t.co/4lTwTF0Mfk

— Kid Miniatures (@kid_miniatures) June 17, 2024