Sammael (usually spelled "Samael," but I greatly prefer the double M spelling variant) is one of the archdevils in Talmudic and other Judeo-Christian lore, sometimes mistaken for Lucifer the Satan (though the two are separate entities). Various sources conflict on whether Sammael is a fallen angel or remains an angel in good standing (more specifically, an angel of death) - or somehow manages to be
both an angel and a devil.
Sammael is the Archdevil of Venom, the Serpent (who tempted Eve, among other things), and Lilith is sometimes named as his consort.
Dicefreaks Gates of Hell (Volume [actually chapter] 8) provides a nice (if not totally accurate, IMO

) D&D version of Sammael, as one of the original Hellspawn (like Dispater, Mephistopheles, Lilith, etc), and former Lord of the Seventh who was deceived and betrayed by Beelzebub. GoH Sammael escaped from Baator by shifting his Venal Fortress to the Gray Waste, and he is now supposedly closer to NE than LE. I think we can safely disregard that particular notion.