Mermail Abyssteus (SHVA-EN039) [Shadows in Valhalla]

You can discard 1 other WATER monster to the GY; Special Summon this card from your hand. If Summoned this way: 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.