Charybdis and Scylla, the sirens were certainly unnatural creatures who could lure men to death through their song. Failing luring by song Charybdis would induce her deadly maelstrom, or whirlpool three times a day and Scylla was a hideous twelve headed beast that would take one man for each of her heads of any passing ship. In order for Odysseus' men to avoid the maelstrom of Charybdis they had to steer far away from her directly into the path of Scylla, right into their trap. This is certainly not an occurrence one would find in nature.
I'd prefer to say "supernatural" rather than unnatural--although a one-eyed giant is certainly unnatural. Since you've included Circes as a supernatural, you probably should include Tiresias as well. Although blind, Tiresias is a seer and can see into the future.
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