From Daniel 2:43 (New King James)
43 As you saw iron mixed with ceramic clay, they will mingle with the seed of men; but they will not adhere to one another, just as iron does not mix with clay.
Dan. 2:40 And the fourth kingdom shall be as strong as iron, inasmuch as iron breaks in pieces and shatters everything; and like iron that crushes, that kingdom will break in pieces and crush all the others. 41 Whereas you saw the feet and toes, partly of potter’s clay and partly of iron, the kingdom shall be divided; yet the strength of the iron shall be in it, just as you saw the iron mixed with ceramic clay. 42 And as the toes of the feet were partly of iron and partly of clay, so the kingdom shall be partly strong and partly fragile. 43 As you saw iron mixed with ceramic clay, they will mingle with the seed of men; but they will not adhere to one another, just as iron does not mix with clay. (NJKV)
The word translated "mingle" means "mix". The clay (= the seed of men) mixes with iron (="they"). Many of the Google search results for this verse claim that the contrast between "they" and "seed of men" indicates that the "they" cannot be human. Thus the "mingling" is the sexual relationship between demons and humans, as in Gen 6:1-4. I do not believe this conclusion is warranted. The word translated "mingling" does not mean sexual contact, but simply a mixing, as the mixing of iron with clay. Rather, as the gold, silver, and bronze all represent humans, so do the iron and clay.
John Gill offers the following explanation:
they shall mingle themselves with the seed of men; the Romans shall mix with people of other and many nations that shall come in among them, and unite in setting up kingdoms; or these kingdoms set up shall intermarry with each other, in order to strengthen their alliances, and support their interests: thus France, Spain, Portugal, and other nations; those of the royal families marry with each other, with such views:
but they shall not cleave one to another, even as iron is not mixed with clay; and yet these ties of marriage and of blood shall not cause them to cleave to and abide by one another; but ambition and worldly interests will engage them to take part with each other's enemies, or to go to war with one another, to the weakening and hurting each other; and thus the potsherds of the earth will dash one another to pieces; and those who are more powerful, like the iron, will trample the weaker like miry clay under their feet.
Whether or not one agrees with the details of Gill's applications of this prophecy to specific nations and intermarriages, his basic interpretation remains solid. In my opinion, anyway.
Some of those who believe that the book of Revelation is future yet to be fulfilled link the "seeds of men" with and Satan and the angels who are cast to earth on the basis of Revelation 12:7-12
And war broke out in heaven: Michael and his angels fought with the dragon; and the dragon and his angels fought, but they did not prevail, nor was a place found for them in heaven any longer. So the great dragon was cast out, that serpent of old, called the Devil and Satan, who deceives the whole world; he was cast to the earth, and his angels were cast out with him.
Then I heard a loud voice saying in heaven, “Now salvation, and strength, and the kingdom of our God, and the power of His Christ have come, for the accuser of our brethren, who accused them before our God day and night, has been cast down. And they overcame him by the blood of the Lamb and by the word of their testimony, and they did not love their lives to the death. Therefore rejoice, O heavens, and you who dwell in them! Woe to the inhabitants of the earth and the sea! For the devil has come down to you, having great wrath, because he knows that he has a short time.”
This is believed to be of a future fall.
This view is also supported by Revelation 16
And I saw three unclean spirits like frogs come out of the mouth of the dragon, and out of the mouth of the beast, and out of the mouth of the false prophet. For they are the spirits of devils, working miracles, which go forth unto the kings of the earth and of the whole world, to gather them to the battle of that great day of God Almighty. Behold, I come as a thief. Blessed is he that watcheth, and keepeth his garments, lest he walk naked, and they see his shame. And he gathered them together into a place called in the Hebrew tongue Armageddon.
So from a futurist literal point of view there will be definitely other beings on earth during this time.....which then concurs with Daniel.
Last but not least pseudepigraphal books such as Enoch mention other entities on the scene during end times:
LVI. 5-8. Last struggle of heathen Powers against Israel.
- And in those days the angels shall return And hurl themselves to the east upon the Parthians and Medes: They shall stir up the kings, so that a spirit of unrest shall come upon them, And they shall rouse them from their thrones, That they may break forth as lions from their lairs, And as hungry wolves among their flocks.
- And they shall go up and tread under foot the land of His elect ones, p. 74 [And the land of His elect ones shall be before them a threshing-floor and a highway:] 7 But the city of my righteous shall be a hindrance to their horses.
And 2 Baruch 27:1-14 appears to describe these entities as well:
And He answered and said unto me: 'Into twelve parts is that time divided, and each one of them is reserved for that which is appointed for it. In the first part there shall be the beginning of commotions. And in the second part (there shall be) slayings of the great ones. And in the third part the fall of many by death. And in the fourth part the sending of the sword. And in the fifth part famine and the withholding of rain. And in the sixth part earthquakes and terrors. [Wanting.] And in the eighth part a multitude of specters and attacks of the Shedim. And in the ninth part the fall of fire. And in the tenth part rapine and much oppression. And in the eleventh part wickedness and unchastity. And in the twelfth part confusion from the mingling together of all those things aforesaid. For these parts of that time are reserved, and shall be mingled one with another and minister one to another.
As such, these "seeds of men" appear to be some type of creature strange creature mentioned in the several writings which discuss the end times.