Atlantean Heavy Infantry (SDRE-EN004)

During your Main Phase, you can Normal Summon 1 Level 4 or lower Sea Serpent-Type monster in addition to your Normal Summon/Set. (You can only gain this effect once per turn.) When this card is sent to the Graveyard to activate a WATER monster's effect: Target 1 face-up card your opponent controls; destroy that target.

Sea Serpent / Effect, Realm of the Sea Emperor Structure Deck