Tantalus the Younger was Clytemnestra's first husband in Greek mythographic tradition. Apollodorus makes him a son of Thyestes whom Agamemnon killed, together with the newborn child of the marriage, before wedding the widow and taking the Mycenaean throne; Pausanias also knows a parentage from Broteas.
Adapted from Wikipedia, "Tantalus" and "Clytemnestra" · CC BY-SA 4.0
I.First marriage of Clytemnestra
In Apollodorus' Epitome, Agamemnon kills Tantalus son of Thyestes and the infant born to Tantalus and Clytemnestra, then marries Clytemnestra himself. The murder clears the Mycenaean succession for the Atreid house and plants another grievance inside the marriage that ends in Agamemnon's murder.
Pausanias preserves a variant in which this Tantalus is son of Broteas rather than Thyestes — a parentage fork the atlas records without collapsing the two stems.
II.Homonyms
This node (tantalus-clytemnestra-husband) is Clytemnestra's first husband only. He is not Tantalus of Sipylus, father of Pelops (tantalus); nor the infant son of Thyestes butchered at Atreus' banquet (tantalus-infant). Same name, three distinct Pelopid figures.