​​AEGISTHUS

Aegisthus was born for the purpose of killing Atreus for feeding Thyestes (Aegisthus' father) his own sons. Thyestes slept with his daughter, Pelopia,  to give life to Aegisthus, per the prophecy Thyestes was told. 

After Atreus is slain, Aegisthus and Thyestes ruled Mycenae together by exiling Atreus' sons, Agamemnon and Menelaus to Sparta. In Sparta is where King Tyndareus gave the two sons his two daughters to marry.
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House of Atreus Family Tree

​https://www.history.com/.image/ar_16:9%2Cc_fill%2Ccs_srgb%2Cfl_progressive%2Cg_faces:center%2Cq_auto:good%2Cw_768/MTU3ODc5MDg3MjQ1MTA4NTUz/the-wall-around-the-tombs-of-grave-circle-a.jpg
https://mythworld.fandom.com/wiki/Aegisthus
​https://www2.classics.upenn.edu/myth/php/tragedy/index.php?page=atreus