Please tell me what Kenites are? I've heard that they are descendents of a union between Satan and Eve.

The Kenites are first mentioned in Genesis 15:18In the same day the LORD made a covenant with Abram, saying, Unto thy seed have I given this land, from the river of Egypt unto the great river, the river Euphrates:
See All.... Their land was among those promised to Abram. Thus, they existed before him. His seed and theirs likely mixed with theirs through Midian, a son that Keturah bare him (Genesis 25:1-2 [1] Then again Abraham took a wife, and her name was Keturah. [2] And she bare him Zimran, and Jokshan, and Medan, and Midian, and Ishbak, and Shuah.
See All...). The Kenites show up later when we meet Jethro. He was the Midianite priest,  who was Moses' father-in-law. (Judges 1:16And the children of the Kenite, Moses' father in law, went up out of the city of palm trees with the children of Judah into the wilderness of Judah, which lieth in the south of Arad; and they went and dwelt among the people.
See All...) Jethro is also called "Hobab," the son of Raguel in Judges 4:11Now Heber the Kenite, which was of the children of Hobab the father in law of Moses, had severed himself from the Kenites, and pitched his tent unto the plain of Zaanaim, which is by Kedesh.
See All... and Numbers 10:29And Moses said unto Hobab, the son of Raguel the Midianite, Moses' father in law, We are journeying unto the place of which the LORD said, I will give it you: come thou with us, and we will do thee good: for the LORD hath spoken good concerning Israel.
See All.... He is also called "Reuel" in Exodus 2:18And when they came to Reuel their father, he said, How is it that ye are come so soon to day?
See All....

The Kenites had a very special place in Israel's history. They traveled with the children of Israel in the desert and therefore identified with them. Through Moses, you might say they are the first cousins of Israel, and enjoyed many of their blessings. Balaam said of them "Strong is thy dwellingplace, and thou puttest thy nest in a rock."  We are familiar of the story of Jael (the wife of Heber the Kenite,) who put the tent nail through the temples of Sissera, the captain of the Canaanite host. (Judges 4:21Then Jael Heber's wife took a nail of the tent, and took an hammer in her hand, and went softly unto him, and smote the nail into his temples, and fastened it into the ground: for he was fast asleep and weary. So he died.
See All...) Her husband's name "Heber" means "Friend" in Hebrew. (The name "Hebrew" was from the same root. It was given to  Abraham, the friend of God.)

When Saul went to slay the Amalekites, he told the Kenites to depart lest any of them were destroyed also (1 Samuel 15:6And Saul said unto the Kenites, Go, depart, get you down from among the Amalekites, lest I destroy you with them: for ye shewed kindness to all the children of Israel, when they came up out of Egypt. So the Kenites departed from among the Amalekites.
See All...).  In 1Samuel 30:29
See All..., David sends the spoils of war to the cities of Judah and the Kenites. In 1 Chronicles 2:55And the families of the scribes which dwelt at Jabez; the Tirathites, the Shimeathites, and Suchathites. These are the Kenites that came of Hemath, the father of the house of Rechab.
See All..., the father of the house of Rechab was a Kenite, who was among the families of scribes.  From that point on, the Kenites were reckoned in the house of Rechab and were called Rechabites.

In 2 Kings 10:15And when he was departed thence, he lighted on Jehonadab the son of Rechab coming to meet him: and he saluted him, and said to him, Is thine heart right, as my heart is with thy heart? And Jehonadab answered, It is. If it be, give me thine hand. And he gave him his hand; and he took him up to him into the chariot.
See All..., Jehu went to his friend Jehonadab, the son of Rechab, asking if his heart was right. The two of them wiped out the wicked religion of Baalism together in northern Israel.