" Because ye have said: `We have made a covenant with death, And with Sheol we have made a provision, An overflowing scourge, when it passeth over, Doth not meet us, Though we have made a lie our refuge, And in falsehood have been hidden.'" (YLT)
Covenant from Strong's Hebrew 1285, a promise, an agreement, an alliance, a treaty
Brown-Driver-Briggs : a pact, compact, covenant :
I. between men, 1. treaty, alliance. league
" Lo, a mighty and strong one [is] to the Lord, As a storm of hail -- a destructive shower, As an inundation of mighty waters overflowing, He cast down to the earth with the hand." (YLT)
The backdrop was the prophesy of the downfall of northern 10 tribes of Israel, referred to as Ephraim collectively (1 Kings 12:25; Zech 10:7), because of their strength and pride as warriors. The "crown of pride" in verse 1 is probably a reference to the beauty of the capital city, Samaria. In judgment language, being cast down to earth was the overthrow of a nation or king. They would no longer rule.
Israel had turned from God to worship idols, Baal worship (2 Kings 22:51-52).
Whether literally drunk with wine, or whether God is sarcastically referring to being drunk with their own sense of power, they were relying upon themselves and upon their allegiances with other men / nations, and not upon God.
Because of their fear of the rising power of the Assyrians in the north, the kings of the 10 northern tribes (Israel, or Ephraim) relied upon treaties with the Assyrians under Tilgath-Pileser, and were paying tribute.
Excerpt from "Old Testament History of Assyrian Dominance" by Dennis Bratcher:
"Faced with the prospect of Assyrian invasion, Menahem taxed the wealthy of the Kingdom to pay tribute to Tiglath-Pileser, thus avoiding a direct invasion. But by so doing, for all practical purposes he had surrendered the nation to the Assyrians and made it a vassal state of the Assyrian Empire." Source: here
Eventually, in desperation even Judah made a treaty with the weakened nation of Egypt for aid in case of an invasion from the Assyrian empire.
"Ahaz, unwilling or unable to wage campaigns on three fronts, began seeking military alliances with other nations. The prophet Isaiah desperately pleaded with Ahaz to trust in the promises of God and not to pursue such a reckless course of action (Isa 7-8). But Ahaz ignored Isaiah, and after overtures to Egypt failed to produce any results, he finally appealed to the Assyrian ruler Tiglath Pileser III for assistance (1 Kings 16:7-10). In effect, Ahaz had willingly surrendered the Southern Kingdom to Assyria." Ibid.
Excerpt from Barne's Notes on the Bible on verse 15:
"We have made a covenant with death - We are not to suppose that they had formally said this, but that their conduct was as if they had said it; they lived as securely as if they had entered into a compact with death not to destroy them, and with hell not to devour them. The figure is a very bold one, and is designed to express the extraordinary stupidity of the nation. It is most strikingly descriptive of the great mass of people. They are as little anxious about death and hell as if they had made a compact with the king of terrors and the prince of darkness not to destroy them. They are as little moved by the appeals of the gospel, by the alarms of God's providence, by the preaching of his word, and by all the demonstrations that they are exposed to eternal death, as though they had proved that there was no hell, or had entered into a solemn covenant that they should be unmolested. A figure similar to this occurs in Job 5:23 :
For thou shalt be in league with the stones of the field;
And the beasts of the field shall be at peace with thee."
Excerpt from The Pulpit Commentary:
"Verse 15. - We have made a covenant with death (comp. Job 5:23; Hosea 2:18). The words are a boast, expressed somewhat enigmatically, that they have secured their own safety by some secret agreement. The exact nature of the agreement they are disinclined to divulge. With hell are we at agreement. A "synonymous parallelism," merely strengthening the previous assertion. When the overflowing scourge shall pass through. Assyrian invasion has been compared to a "flood" (Isaiah 8:7; Isaiah 28:2), and to a "rod" or "staff" in Isaiah 10:24. Here the two metaphors are joined together. It shall not come to us. Some means will be found - what, they do not say, either for diverting the flood, or for stemming it. For we have made lies our refuge. Here the Divine reporter departs from the language of those whose words he is reporting, and substitutes his own estimate of the true nature and true value of that "refuge" on which they placed such entire reliance. It appears by Isaiah 30:1-7 and Isaiah 36:6-9 that that refuge was Egypt. Now, Egypt was a "bruised reed," not to be depended on for keeping her engagements. To trust in her was to put confidence in "lies" and "falsehood."" Ibid.
Instead of trusting in God, they put their trust in the strength of men, or the perceived strength of nations. These were promises they could not keep, and as they were promoted by the false prophets, they were promises of lies... covenants with death.
God equates lying with murder (John 8:44). Lies equal death in God's eyes.
The strong one... the king of Assyria swept over the land like a flood, and captured Samaria, the capital of Israel, and took the 10 northern tribes into captivity around 722 BC.