Genesis 38

Judah (the brother who sold Joseph into slavery) moved to a different city. He married an unnamed Canaanite and had three male children with her. He found Tamar for his first son, Er, who was so wicked that the Lord slew him before he could reproduce.

Judah enforced his second son Onan to uphold the tradition. He was slain for engaging in sexual activity with Tamar, but intentionally avoided pregnancy. This tradition would be credited to Er instead of him, and he did not want to uphold that tradition.

Judah promised his third son, Selah would become Tamar's husband when he was of age, but did not enforce it.

Judah is tricked by Tamar into fathering twin boys with her.