Nephilim are mentioned twice in the Bible. The first time was in Genesis 6:
Presumably, they were extinguished in Genesis 7:4 The Nephilim were on the earth in those days—and also afterward—when the sons of God went to the daughters of humans and had children by them. They were the heroes of old, men of renown.
Nephilim reappeared in Numbers 13:23 Every living thing on the face of the earth was wiped out; people and animals and the creatures that move along the ground and the birds were wiped from the earth. Only Noah was left, and those with him in the ark.
Presumably, they made a reappearance the second time as they did the first time when the sons of God went to the daughters of humans and had children by them.33 We saw the Nephilim there (the descendants of Anak come from the Nephilim). We seemed like grasshoppers in our own eyes, and we looked the same to them.