Question:What happened to Ishmael's descendants?
God promised Abraham, "And as for Ishmael, I have heard you. Behold, I have blessed him, and will make him fruitful, and will multiply him exceedingly. He shall beget twelve princes, and I will make him a great nation" (Genesis 17:20).
Ishmael's initial descendants are recorded for us in the Bible: "Now this is the genealogy of Ishmael, Abraham's son, whom Hagar the Egyptian, Sarah's maidservant, bore to Abraham. And these were the names of the sons of Ishmael, by their names, according to their generations: The firstborn of Ishmael, Nebajoth; then Kedar, Adbeel, Mibsam, Mishma, Dumah, Massa, Hadar, Tema, Jetur, Naphish, and Kedemah. These were the sons of Ishmael and these were their names, by their towns and their settlements, twelve princes according to their nations" (Genesis 25:12-16).
One of Ishmael's daughters, Mahalath, married Isaac's son, Esau (Genesis 28:9). She was Esau's third wife. Esau and his descendants eventually founded the nation of Edom, which is named after Esau's nickname.
Most of Ishmael's descendants founded the Arab nations. According to Bill Cooper, in his book After the Flood, 1995 (p. 183-185):
Nebaioth: He settled with his descendants to the south of the Dead Sea, where they were known to the Chaldeans as the Nabat, and to the Assyrians as the Nabaiate. Their own inscriptions render the naem as 'nbtw.' The Greek historian, Diodorus, mentions them, and Ptolemy knew them as the Nabatei. The Nabataeans' final demise was brought about by Augustus Caesar, who cut off the trade routes of Arabia. By the time of Tiberius Caesar, all the land east of Judea was known as Nabataea.
Kedar: Known to the Hebrews as the Qedar, and the Assyrians as the Qidri, his descendants became the great tribe of Arabs who settled in the northwest Arabian peninsula, and whose black tents were to become proverbial in the ancient world. We are informed in Babylonian sources that the armies of Nebuchadnezzar confronted the tribe of Kedar in a major skirmish of the year 599 BC, an incident that was foretold by Jeremiah (49:28 and 29). The tribe of Kedar is also mentioned in the annals of Ashurbanipal, with whom they clashed, and in various other Assyrian documents. In these, the men of Kedar are mentioned in close association with the men of Nebaioth. The founder of Islam, Mohammed, was to trace his own direct descent from Kedar.
Adbeel: He was the founder of a tribe who were known to the Akkadians as the Idibilu. This same people were subsequently mentioned in the annals of Tiglath-pileser III, who tells us how he conquered the Idiba'leans and employed them to guard the approaches to Egypt's borders. Their area of settlement was in northwest Arabia, close to the lands of Kedar and Nebaioth.
Mibsam: An otherwise unknown Bedhouin chieftain.
Mishma: He settled with his descendants in what is known today as Jebel Mishma in the vicinity of Tema.
Dumah: The Assyrians and Babylonians knew Dumah's descendants as the Adammatu. Nabonidus later tells us how he conquered the Adummu. Ptolemy referred to them as the Domatha; and Porphyry recorded their name as the Dumathi. We know them today as the Idumaeans. The name of Dumah is still preserved in the modern Arab city of Dumat-al-Jandal, the erstwhile capital of his tribe.
Massa: The descendants of Massa were known to the Assyrians as the Mas'a, who with the tribe of Tema were forced to pay tribute to Tiglath-pileser III. He tells us how he conquered them along with the peoples of Haiappa, the Idiba'leans and others. Ptolemy knew the tribe as the Masanoi, who lived to the northeast of Dumah. Josepheus records their name as Mesanaeans, and that in his day their lands were known to the Romans as Charax Spasini.
Hadad: The name is rendered as Haddu in Akkadian inscriptions as the name of a pagan god. Hadad himself, however, seems to be unknown in extra-biblical sources.
Tema: Still known by today's Arabs as Taima', the city of Tema's descendants lies some 70 miles northeast of Dedan. Nabonidus, king of Babylon, (556-539 BC), passed his years of exile in this city, which he also knew as Tema. The city of Tema, with those of Dedan and Dumah, formed stages in the caravan route from Babylon to Sheba.
Jetur: he was the progenitor of the Ituraeans, who were known to the Greeks as the Itouraia. The Ituraeans are mentioned in the works of Dio Cassius, Josephus, Pliny, Strabo and others; and were known to the Roman authorities as a tribe of robbers. The descendants of Jetur perpetrated a massacre of Lebanese Christians in AD 1860.
Naphish: He and his lineage are variously known in the biblical records as Nephish, the children of the Nephusim, and the Nephishesim. They are seemingly unknown from extra-biblical sources.
Kedemah: He and his descendants settled in what was later known as the Wilderness of Kedemoth. The tribe dwelt in the city that is known today as es-Za'feran.
I recommend the following book if you are interested in where various nations and people mentioned in the Bible settled, especially those found in the Table of Nations:
After the Flood, Bill Cooper, B.A. Hons., New Wine Press, 1995, ISBN: 187436740X.