Onan

Second son of Judah by his un-named Canaanite wife. By the ancient custom of levirate marriage he was required to marry his brother's widow, Tamar and sire a son who would inherit his brother's name. Instead he spill his semen during copulation "to keep from producing offspring for his brother" (Gen 38:9). For this wickedness Yahweh put him to death, becoming the second person in the biblical history of humankind to be specifically put to death by Yahweh (his older brother Er was the first). His death led to the complicated events that eventually brought Judah two sons—Perez and Zerah—by his daughter-in-law Tamar. We know nothing else about Onan.

CONCORDANCE (NIV)

Gen 38:4 — She [Judah's wife] conceived again and gave birth to a son and named him Onan.

Gen 38:8-9 — Then Judah said to Onan, "Lie with your brother's wife and fulfil your duty to her as a brother-in-law to produce offspring for your brother." But Onan knew that the offspring would not be his; so whenever he lay with his brother's wife, he spilled his semen on the ground to keep from producing offspring for his brother.

Gen 46:12 — The sons of Judah: Er, Onan, Shelah, Perez and Zerah (but Er and Onan had died in the land of Canaan). The sons of Perez: Hezron and Hamul.

Num 26:19 — Er and Onan were sons of Judah, but they died in Canaan.

1 Chron 2:3 — The sons of Judah: Er, Onan and Shelah. These three were born to him by a Canaanite woman, the daughter of Shua. Er, Judah's firstborn, was wicked in the Lord's sight; so the Lord put him to death.