The Sea Mine

The Sea Mine is a Hardmode flail that is sold by the Diverman after Coznix, The Fallen Beholder is defeated. If it manages to deal five hits of damage in one throw, the fifth hit will unleash a fiery explosion that expands outwards briefly, dealing great damage upon and inflicting the On Fire! debuff to all enemies caught in the blast.

The flail head can be extended up to 17 tiles before returning. Like most flails, you can hold down the mouse button to flail it around as you walk.