(Leechridden Seafloor isn't a spell.)

At the beginning of your end step, if you didn't cast any spells this turn, Leechridden Seafloor deals 1 damage to you.

Leechridden Seafloor is indestructible. (Destroy effects and lethal damage don't destroy it.)

: Add  or to your mana pool. Leechridden Seafloor deals 2 damage to you.