The offspring of the union of the Watchers with the daughters of men were the giants. Said to be 300 cubids tall, the nephilim were eventually dispatched by God’s executive order, much like the titanic Titans of Greek mythology. The fire-giants of Norse mythos were also aligned with the powers of darkness.
“And the women became pregnant and gave birth to great giants whose heights were three hundred cubits. These (giants) consumed the produce of all the people until the people detested feeding them. So the giants turned against (the people) in order to eat them." - 1 Enoch 7:1-5
"For it was on account of these three things [fornication, uncleanness, and injustice - see Jubilees 7:20] that the flood was on the earth, since (it was) due to fornication that the Watchers had illicit intercourse - apart from the mandate of their authority - with women. When they married of them whomever they chose they committed the first (acts) of uncleanness. They fathered (as their) sons the Nephilim. - Jubilees 7:21-22
The race of giants perished in the Flood, their disembodied souls wandered through the world as the race of demons. These spirits are not material or corporal beings, but they torment mankind because they have proceeded from them.
According to 1 Enoch, these spirits will not be punished until the day of judgment, in contrast to the Watchers, who are punished both before and on the day of judgment.
"But now the giants who are born from the (union of) the spirits and the flesh shall be called evil spirits upon the earth, because their dwelling shall be upon the earth and inside the earth. Evil spirits have come out of their bodies. Because from the day that they were created from the holy ones they became the Watchers; their first origin is the spiritual foundation. They will become evil upon the earth and shall be called evil spirits. The dwelling of the spiritual beings of heaven is heaven; but the dwelling of the spirits of the earth, which are born upon the earth, is in the earth. The spirits of the giants oppress each other, they will corrupt, fall, be excited, and fall upon the earth, and cause sorrow. They eat no food, nor become thirsty, nor find obstacles. And these spirits shall rise up against the children of the people and against the women, because they have proceeded forth (from them). - 1 Enoch 15
2 Enoch also mentions a group of angels called the Grigori, who are similar to the Watchers. Their prince is called Satanail. A difference in this account as compared with the two previous accounts is that only 3 angels came down to earth to take wives and beget giants.
"These are the Grigori, who with their prince Satanail rejected the Lord of light, and after them are those who are held in great darkness on the second heaven, and three of them went down on earth to the place Ermon, and broke through their vows on the shoulder of the hill Ermon and saw the daughters of men how good they are, and took to themselves wives, and befouled the earth with their deeds, who in all times of their age made lawlessness and mixing, and giants are born and marvellous big men and great enmity. And therefore God judged them with great judgment, and they weep for their brethren and they will be punished on the Lord's great day." - 2 Enoch 18:3-4