Paul Harvey, ed. The Oxford Companion to Classical Literature
(1937; rpt. 1969), s.v. Palladium:
In Greek and Roman belief, an image of immemorial antiquity on which the safety of a city was thought to depend. It was said to be the image of Pallas, whom the Greeks identified with Athene and the Romans with Minerva, and to have fallen from heaven in answer to the prayer of Ilus the founder of Troy. Since Troy could not be captured while it contained this image, Diomedes and Odysseus (or Diomedes alone) carried it off. According to various versions, it found its way to Athens, or Argos, or Sparta, or Rome. To this last city it was either brought by Aeneas (Diomedes having only succeeded in stealing an imitation) or surrendered by Diomedes. It was kept there in the temple of Vesta.
Matthew Arnold, Palladium
Set where the upper streams of Simois flow
Was the Palladium, high 'mid rock and wood;
And Hector was in Ilium, far below,
And fought, and saw it not - but there it stood!
It stood, and sun and moonshine rain'd their light
On the pure columns of its glen-built hall.
Backward and forward roll'd the waves of fight
Round Troy - but while this stood, Troy could not fall.
So, in its lovely moonlight, lives the soul.
Mountains surround it, and sweet virgin air;
Cold plashing, past it, crystal waters roll;
We visit it by moments, ah, too rare!
We shall renew the battle in the plain
To-morrow; - red with blood will Xanthus be;
Hector and Ajax will be there again,
Helen will come upon the wall to see.
Then we shall rust in shade, or shine in strife,
And fluctuate 'twixt blind hopes and blind despairs,
And fancy that we put forth all our life,
And never know how with the soul it fares.
Still doth the soul, from its lone fastness high,
Upon our life a ruling effluence send.
And when it fails, fight as we will, we die;
And while it lasts, we cannot wholly end.