Judges 3:13 (ESV)

13 He gathered to himself the Ammonites and the Amalekites, and went and defeated Israel. And they took possession of the city of palms.

Amalek was one of the chiefs descended from Eliphaz, firstborn of Esau (Genesis 36:15–16. His descendants lived mostly in the Negev (Numbers 13:29). He is consistently presented as a fierce enemy of Israel and therefore of Yahweh himself, and thus a memory to be blotted out (Exodus 17:14; Deuteronomy 25:19). Interestingly, the Amalekites were the first enemy Israel defeated after the exodus (Exodus 17:8–13). Now the tables are turned, to Israel’s humiliation.