Here is the stanza you're asking about:

    On what foundation stands the warrior's Pride?
How just his hopes let Swedish Charles decide;
A frame of adamant, a soul of fire,
No Dangers fright him, and no labours tire;
O'er love, o'er force, extends his wide domain,
Unconquered lord of pleasure and of pain;
No joys to him pacific scepters yield,
War sounds the trump, he rushes to the field;
Behold surrounding kings their power combine,
And one capitulate, and one resign...
...............................
"Think Nothing gained," he cries, "till nought remain,
On Moscow's walls till Gothic standards fly,
And all is mine beneath the polar sky."...
...............................
Did rival monarchs give the fatal wound?
Or hostile millions press him to the ground?
His fall was destined to a barren strand,
A petty fortress, and a dubious hand;
He left the name, at which the world grew pale,
To point a moral, or adorn a tale.

Charles XII was King of Sweden from 1697 to 1718. During his reign Sweden was involved in the Great Northern War with Peter the Great of Russia. Before that war, Sweden was the leading power in the Baltic, but Charles's army was defeated in 1718 and he was forced to go into exile.

Johnson is telling us that despite how heroically Charles began his battles, and regardless of his bravado in taunting Moscow, fate decreed a different outcome for him: "To point a moral, or adorn a tale."