Here's a couple that I copied from The Celts: A History, by Dáithí O hOgain. There is a concordance of citations in the book: I'll hunt them up on Perseus and add in the original text where possible.

But Poseidonius, in the twenty-third book of his Histories, says, "The Celts sometimes have single combats at their entertainments. For having assembled in arms, they go through the exercise, and make feints at, and sometimes they even go so far as to wound one another. And being irritated by this, if the bystanders do not stop them, they will proceed even to kill one another. But in olden times," he continues, "there was a custom that a hind quarter of pork was put on the table, and the bravest man took it; and if any one else laid claim to it, then the two rose up to fight till one of them was slain".
Quoted in Athenaeus: The Deipnosophists

There is also that custom, barbarous and exotic, which attends most of the northern tribes — I mean the fact that when they depart from the battle they hang the heads of their enemies from the necks of their horses, and, when they have brought them home, nail the spectacle to the entrances of their homes. At any rate, Poseidonius says that he himself saw this spectacle in many places, and that, although at first he loathed it, afterwards, through his familiarity with it, he could bear it calmly. The heads of enemies of high repute, however, they used to embalm in cedar-oil and exhibit to strangers, and they would not deign to give them back even for a ransom of an equal weight of gold.
Strabo, 4.4.5

When they are stirred up they assemble in their bands for battle, quite openly and without forethought, so that they are easily handled by those who desire to outwit them; for at any time or place on whatever pretext you stir them up, you will have hem ready to face danger, even if they have nothing on their side but their own strength and courage... Their strength depends on their mighty bodies, and on their numbers.
Source to be found (Poseidonius)

They rushed on their enemy with the rage and fury and blind courage of a wild beast. Hacked with swords and axes, and pierced with darts and javelins, their fury only died with life itself. Indeed, some even plucked out the weapons that struck them and hurled them back at the Greeks, or used them in hand-to-hand fighting.
Hieronymus of Cardia


οὐ γὰρ θεὸν εἶναι τὸν ἐπιόντα ἐπὶ τὴν Ἑλλάδα ἀλλ' ἄνθρωπον, εἶναι δὲ θνητὸν οὐδένα οὐδὲ ἔσεσθαι τῷ κακὸν ἐξ ἀρχῆς γινομένῳ οὐ συνεμίχθη, τοῖσι δὲ μεγίστοισι αὐτῶν μέγιστα

The invader of Hellas was not a god but a human being, and there was not, and never would be, any mortal on whom some amount of evil was not bestowed at birth, with the greatest men receiving the largest share. The one marching against them was certain to fall from pride, since he was a mortal.

Herodotus, 7.203

ἀνὴρ ἄριστος γενέσθαι Σπαρτιήτης Διηνέκης: τὸν τόδε φασὶ εἰπεῖν τὸ ἔπος πρὶν ἢ συμμῖξαι σφέας τοῖσι Μήδοισι, πυθόμενον πρός τευ τῶν Τρηχινίων ὡς ἐπεὰν οἱ βάρβαροι ἀπίωσι τὰ τοξεύματα, τὸν ἥλιον ὑπὸ τοῦ πλήθεος τῶν ὀιστῶν ἀποκρύπτουσι: τοσοῦτο πλῆθος αὐτῶν εἶναι. [2] τὸν δὲ οὐκ ἐκπλαγέντα τούτοισι εἰπεῖν ἐν ἀλογίῃ ποιεύμενον τὸ Μήδων πλῆθος, ὡς πάντα σφι ἀγαθὰ ὁ Τρηχίνιος ξεῖνος ἀγγέλλοι, εἰ ἀποκρυπτόντων τῶν Μήδων τὸν ἥλιον ὑπὸ σκιῇ ἔσοιτο πρὸς αὐτοὺς ἡ μάχη καὶ οὐκ ἐν ἡλίῳ.

The Spartan Dieneces is said to have exhibited the greatest courage of all. They say that he made the following speech before they joined battle with the Medes: he had learned from a Trachinian that there were so many of the barbarians that when they shot their missiles, the sun was hidden by the multitude of their arrows. He was not at all disturbed by this and made light of the multitude of the Medes, saying that their Trachinian foreigner brought them good news. If the Medes hid the sun, they could fight them in the shade instead.

Herodotus, 7.226


Of course this is all raw material and will have to be edited to fit the loading screen.

Oh and thanks, Machinor. I'll get to work on that.