The Nephilim

Genesis Chapter 6, verses 1 through 4 mentions Nephilim:

Now it came about, when men began to multiply on the face of the land, and daughters were born to them, that the sons of God saw that the daughters of men were beautiful; and they took wives for themselves, whomever they chose. Then the LORD said, "My Spirit shall not strive with man forever, for he is indeed flesh; nevertheless his days shall be one hundred and twenty years." The Nephilim were on the earth in those days, and also afterward, when the sons of God came in to the daughters of men, and they bore children to them. Those were the mighty men who were of old, men of renown.

They are mentioned again in Numbers chapter 13, verses 32-33, in a description of the inhabitants of Hebron:

So they gave out to the sons of Israel a bad report of the land which they had spied out, saying, "The land through which we have gone, in spying it out, is a land that devours its inhabitants; and all the people whom we saw in it are men of great size. There also we saw the Nephilim and we became like grasshoppers in our own sight, and so we were in their sight."