The great storm spell was a great spell that summons storms.
Background[]
All the pirates but one are now dead, the victims of a great storm spell cast upon them by Prince Alexander some weeks after his return home. The lone survivor washed up on the coast of Llewdor babbling of shipmates being consumed by sharks and others swallowed by a great squid after being crushed in its tentacles. The unfortunate man also kept whimpering a word that sounded something like "Cthulhu," but his mind had been shattered by the experience and he only continued to chatter incoherently.
Behind the scenes[]
It's not clear if this is a spell like Storm Brew spell or something far stronger.
It's not clear if the spell itself summoned Cthulhu (something similar to the Iä R'lyea! Cthulhu ftagn! Iä! Iä! spell), or that Cthulhu showing up was a coincidence and that the creature, and others were stirred up by the commotion of a feeding frenzy over the castaways in the water..