Call of Cthulhu Black & Green Dice set: - Rules of Play says:
Imagine yourself, swimming in a deep ocean during the night. The sea is cold and strangely calm... Suddenly, you see inhuman green shapes moving towards you! Just when you think that those unspeakable sea horrors got you ... you wake up. A set of black-green dice lies at your table – and you are sure they weren't there when you fell asleep...