The sea is full of life deserving to become fantasy monsters. I particularly like the benthic and mezopalegic beasts of the abyss and waters just above the abyssal levels - all those phosphorescent creatures. Also fossils of Precambrian sea life is also worthy of becoming RPG monsters - I've used something akin to Crinoids before, all these seem well off as Aberrant beings, somehow related to Aboleth...

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