D&d Cult Of Vecna. In effect these halflings are more vecna victims than vecna cultists. Happy to elaborate more of you want to hear it.
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Iirc Vecna was originally from Oerth but has since appeared in other settings such as FR. Or at least established a cult on Toril. The cult of Vecna is always working to ensure that their wicked god is victorious which makes them great villains to use in a DD campaign.
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The eye looks like a bloodshot organ torn free from the socket. The Serpent appeared in a Living Grehawk core adventure series Windows to the Serpents Soul. 19 rows Cult of Vecna. The Eye of Vecna and the Hand of Vecna might be found together or separately.