Mermail Abyssteus (BLLR-EN051)

You can discard 1 other WATER monster to the GY; Special Summon this card from your hand. When Summoned this way: You can add 1 Level 4 or lower "Mermail" monster from your Deck to your hand. You can only use this effect of "Mermail Abyssteus" once per turn.

Aqua / Effect, Battles of Legend: Light's Revenge