Deeplurker

There are lots of monsters with opportunity but deeplurker is the best and most unique monster with opportunity ability in my opinion. At max stat this card can do 5 melee damage which is a great stat for a monster with opportunity ability and additionally this card gets poison ability which makes him ultimate killer. At max level deeplurker also reduces the melee attack of opponent which makes this card great at game mode where only melee monsters can be used.
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