From the Introduction:
From time immemorial, the rulers of the mortal world have been counseled and tempted by capricious primordial spirits who rule over the elements. The greatest and most sinister of these was a half-kraken medusa known as the Maelstrom Queen, who sought to supplant the mortal races with her own line of immortal tentacled monstrosities. On the verge of her ultimate triumph, when the leviathans at her command rose from the depths, a legendary assassin murdered the Maelstrom Queen in her half-submerged palace with her own weapon. But the primordial mistress refused to let something as trifling as death put an end to her designs.
Every 13 years, she regenerates enough mystic power to rise again, and the world’s greatest heroes are sent to murder the
Maelstrom Queen once more.