Fierce King and a Fierce Nation and a Language Not Understood

Jeremiah 5:15 Lo, I will bring a nation upon you from far, O house of Israel, saith the LORD: it is a mighty nation, it is an ancient nation, a nation whose language thou knowest not, neither understandest what they say.

The nation would be fierce:

Deuteronomy 28:49 The LORD shall bring a nation against thee from far, from the end of the earth, as swift as the eagle flieth; a nation whose tongue thou shalt not understand;

50 A nation of fierce countenance, which shall not regard the person of the old, nor shew favour to the young:

God was prophesying of His intention to raise up the evil empire of Babylon to come against His own people:

Habakkuk 1:6 For, lo, I raise up the Chaldeans, that bitter and hasty nation, which shall march through the breadth of the land, to possess the dwellingplaces that are not theirs.

7 They are terrible and dreadful: their judgment and their dignity shall proceed of themselves.

8 Their horses also are swifter than the leopards, and are more fierce than the evening wolves: and their horsemen shall spread themselves, and their horsemen shall come from far; they shall fly as the eagle that hasteth to eat.

The fierce nation also had, of course, a fierce king reigning over it:

Daniel 8:23 And in the latter time of their kingdom, when the transgressors are come to the full, a king of fierce countenance, and understanding dark sentences, shall stand up.

24 And his power shall be mighty, but not by his own power: and he shall destroy wonderfully, and shall prosper, and practise, and shall destroy the mighty and the holy people.

The fierce king was said to "destroy the mighty and the HOLY people".

Spiritually, the fierce king is Satan. And the fierce nation the unsaved of the kingdom of darkness (of this world). And the holy people to be destroyed by them – the churches and congregations of the world.