The Palmer was originally a bogeyman used to parents to scare their children into submission. "Don’t fuss," they’d say, "or the Palmer will get you!" Parents described the Palmer as a husky, short man with a gleaming bald head; he might even pass for human if his eyes did not register an obvious and burning supernatural malevolence. "Finish your milk, or the Palmer will come in the night and give you poison to drink!"
There is a funny thing about belief: as soon as enough mothers and fathers had filled their progeny’s heads with scare-tales about the fictional bogeyman, he became real because the children believed he was real. Now the Palmer stalks the night, seeking children to poison.
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Stats: use the stats of a Shadow (Monster Manual 269), but replace its necrotic damage with poison damage.
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This monster was inspired by the real-life Victorian murderer, William Palmer. You can read all about him in Stephen Bates’s book Poisoner.