David I was famously the 'Sair sanct for the croun' whose piety led to uniquely-generous royal donations to the monastic orders. Sanctity, however, did not prevent him carrying on that other Scots royal tradition - invading England. Richard of Hexham, a contemporary English chronicler, gives this account of David's less-than-pious military adventuring. By Picts he probably means the Galwegians, who spoke Galloway Gaelic and who must have seemed exotic in the extreme to this English cleric.
And then that execrable army, more atrocious than the whole race of pagans, neither fearing God nor regarding man, spread desolation over the whole province, and murdered everywhere persons of both sexes, of every age and rank, and overthrew, plundered, and burned towns, churches, and houses. For the sick on their couches, women pregnant and in childbed, infants in the womb, innocents at the breast, or on the mother's knee, with the mothers themselves, decrepit old men and worn-out old women, and persons debilitated from whatever cause, wherever they met with them, they put to the edge of the sword, and transfixed with their spears; and by how much more horrible a death they could despatch them, so much the more did they rejoice . . .
That infamous army received accessions from the Normans, Germans, and English, from the Northumbrians and Cumbrians, from Teviotdale and Lothian, from the Picts, commonly called Galwegians, and the Scots, and no one knew their number; for multitudes uncalled-for allied themselves with those above mentioned, either from love of plunder, or opportunity of revenge, or the mere desire of mischief with which that region was rife. Overrunning the province, and sparing none, they ravaged with sword and fire almost all Northumberland as far as the river Tyne, excepting the towns and the sea-coast which lies on the eastern side, but this they designed to devastate on their return . . .
[The English army is mustered.] While thus awaiting the approach of the Scots, the scouts whom they had sent forward to reconnoitre returned, bringing the information that the king with his army had already passed the river Tees and was ravaging their province in his wonted manner: They therefore hastened to resist them; and passing the village of Alverton [North Allerton], they arrived early in the morning at a plain distant from it about two miles. Some of them soon erected, in the centre of a frame, which they brought, the mast of a ship, to which they gave the name of the Standard . . .
Scarcely, then, had they put themselves in battle array, when tidings were brought that the king of Scotland was close at hand with his whole force, ready and eager for the contest. The greater part of the knights, then dismounting, became foot soldiers, a chosen body of whom, interspersed with archers, were arranged in the front rank. The others, with the exception of those who were to dispose and rally the forces, mustered with the barons in the centre, near and around the standard, and were enclosed by the rest of the host, who closed in on all sides. The troop of cavalry and the horses of the knights were stationed at a little distance, lest they should take fright at the shouting and uproar of the Scots. In like manner, on the enemy's side, the king and almost all his followers were on foot, their horses being kept at a distance. In front of the battle were the Picts; in the centre, the king with his knights and English; the rest of the barbarian host poured roaring around them.
As they advanced in this order to battle, the standard with its banners became visible at no great distance; and at once the hearts of the king and his followers were overpowered by extreme terror and consternation; yet, persisting in their wickedness, they pressed on to accomplish their bad ends. On the octaves of the Assumption of St Mary, being Monday, the eleventh of the kalends of September [22 August], between the first and third hours, the struggle of this battle was begun and finished. For numberless Picts being slain immediately on the first attack, the rest, throwing down their arms, disgracefully fled. The plain was strewed with corpses; very many were taken prisoners; the king and all the others took to flight; and at length, of that immense army all were either slain, captured, or scattered as sheep without a shepherd. They fled like persons bereft of reason, in a marvellous manner, into the adjoining district of their adversaries, increasing their distance from their own country, instead of retreating towards it. But wherever they were discovered, they were put to death like sheep for the slaughter; and thus, by the righteous judgement of God, those who had cruelly massacred multitudes, and left them unburied, and giving them neither their country's nor a foreign rite of burial, - left a prey to the dogs, the birds, and the wild beasts, - were either dismembered and torn to pieces, or decayed and putrefied in the open air. The king also, who, in the haughtiness of his mind and the power of his army, seemed a little before to reach with his head even to the stars of heaven, and threatened ruin to the whole or greatest part of England, now dishonoured and meanly attended, barely escaped with his life, in the utmost ignominy and dismay. The power of Divine vengeance was also most plainly exhibited in this, that the army of the vanquished was incalculably greater than that of the conquerors. No estimate could be formed of the number of the slain; for, as many affirm, of that army which came out of Scotland alone, it was computed by the survivors that more than ten thousand were missing; and in various localities of the Deirans, Bernicians, Northumbrians, and Cumbrians, many more perished after the fight than fell in the battle.
Richard of Hexham, 'The Acts of King Stephen and the Battle of the Standard', in Church Historians of England, ed. Joseph Stevenson, iv, pt. i, London, 1856.