Mermail Abyssturge

Per-phrase breakdown

• When this card is Normal or Special Summoned: You can discard 1 WATER monster to the Graveyard, then target 1 Level 3 or lower WATER monster in your Graveyard; add that target to your hand. (Trigger)
• You can only use the effect of "Mermail Abyssturge" once per turn. (Condition)

Judge-facing facts

• Normal Summon / Set from hand uses no tributes (Level 4).
• Trigger Effect that activates when Normal or Special Summoned.
• Discarding 1 WATER monster to the Graveyard is the cost to activate this effect.
• Targets 1 Level 3 or lower WATER monster in your Graveyard at activation.
• You can only use the effect of "Mermail Abyssturge" once per turn.

Card-specific notes from database

• Can be activated during the Damage Step (Konami neuron).
• If a monster discarded as cost is banished from the Graveyard by a chained effect before the chain resolves, its trigger effect cannot be activated (YGOResources).
• If a monster discarded as cost is targeted and added back to the hand by this effect's resolution, that discarded monster's trigger effect cannot be activated (Konami neuron).