What It Is
Pallas's Belt is the baldric, the strap that holds the sword, which Pallas wears on his shoulder when he dies in Book X. It is of gold worked by the craftsman Clonus and bears engraved the massacre of the husbands by the daughters of Danaus, in a single wedding night. Turnus tears it from the corpse and takes to wearing it as a trophy, and the poem returns it at the last moment of Book XII, on the shoulder of the defeated: it is the sight of it that decides the final blow of Aeneas.
Where It Appears
In Book VIII, Evander hands his son over to Aeneas so that he may learn war, and at the farewell laments that he no longer has the strength he once had, which obliges him to give to the one who has it the son he loves most:
Non ego nunc dulci amplexu divellerer usquam, / Nate, tuo; neque finitimo Mezentius unquam / Huic capiti insultans, tot ferro saeva dedisset / Funera, tam multis viduasset civibus urbem, I would not now be torn from your sweet embrace, son; nor would Mezentius, mocking this neighboring head, ever have dealt so many cruel deaths by the sword, nor left the city widowed of so many citizens.
In Book X, Pallas, who knows he does not have Turnus's strength, prays to Hercules, and Turnus runs him through with his spear. After telling the Arcadians that he returns the body to Evander, Turnus tears off the baldric:
rapiens immania pondera baltei, / Impressumque nefas: una sub nocte jugali / Caesa manus juvenum foede thalamique cruenti, snatching the huge weight of the baldric and the crime engraved upon it: a band of young men slaughtered, horribly, in a single bridal night, and the marriage beds full of blood.
At this point the narrator breaks off and speaks of Turnus, announcing that he will hate this spoil:
Nescia mens hominum fati sortisque futurae, / Et servare modum, rebus sublata secundis! / Turno tempus erit, magno cum optaverit emptum / Intactum Pallanta, et cum spolia ista diemque / Oderit, O human mind, ignorant of fate and of the lot to come, and unable to keep measure when lifted by prosperity! A time will come when Turnus will wish he had paid dearly to see Pallas unharmed, and will hate these spoils and that day.
When the news reaches Aeneas, he advances without pause and refuses the ransom of Magus, saying that Turnus abolished the dealings of war when he killed Pallas:
Argenti atque auri memoras quae multa talenta, / Gnatis parce tuis: belli commercia Turnus / Sustulit ista prior, jam tum Pallante perempto. / Hoc patris Anchisae Manes, hoc sentit Iulus, the silver and gold you mention in so many talents, keep for your sons: Turnus was the first to abolish those dealings of war, already when he killed Pallas. This the shades of my father Anchises feel, and Iulus too.
In Book XI, before the body, Aeneas reproaches himself with the faith he gave the father, and Evander, in Pallanteum, sends word that the only reason he has left to live is that Aeneas's hand will bring about Turnus's death. The baldric does not appear in that book, but the debt it will collect is incurred there:
Hi nostri reditus, exspectatique triumphi? / Haec mea magna fides!, is this our return, are these the triumphs we waited for? This is my great faith!
Quod vitam moror invisam, Pallante perempto, / Dextera causa tua est, Turnum gnatoque patrique / Quam debere vides: meritis vacat hic tibi solus / Fortunaeque locus, that I still linger in a hated life, Pallas being slain, is your hand's doing: you see that it owes Turnus to son and to father; this place alone remains for your merits and your fortune.
In Book XII, Turnus, fallen, asks Aeneas for his life or his body, appeals to his father Anchises, and gives him Lavinia before the Ausonians:
Equidem merui, nec deprecor, inquit; / Utere sorte tua. Miseri te si qua parentis / Tangere cura potest, oro (fuit et tibi talis / Anchises genitor), Dauni miserere senectae; / Et me, seu corpus spoliatum lumine mavis, / Redde meis. Vicisti, et victum tendere palmas / Ausonii videre: tua est Lavinia conjux; / Ulterius ne tende odiis, I have earned it, and I do not beg; use your fortune. If any care for a wretched father can touch you (you too had such a father, Anchises), have pity on the old age of Daunus; and return me to my people, or my body stripped of light, if you prefer. You have won, and the Ausonians saw the conquered stretch out his palms: Lavinia is your wife; do not carry hatred any further.
Aeneas was about to yield when the baldric appears. The poem calls it unhappy, and says that Aeneas, seeing on the shoulders of the vanquished that hostile insignia, drinks in with his eyes the monument of cruel grief and the spoil, and is inflamed with fury. He names Pallas twice and assigns the blow to him:
Tune hinc spoliis indute meorum / Eripiare mihi? Pallas te hoc vulnere, Pallas / Immolat et poenam scelerato ex sanguine sumit, you, dressed in the spoils of my people, will escape me from here? Pallas, Pallas, sacrifices you with this wound and exacts the penalty from your criminal blood.
What It Means
The baldric is a trophy that becomes evidence. Turnus wants it because it is rich, worked gold, and he exults in possessing it, and the narrator says a time will come when he will wish he had paid dearly to see Pallas unharmed and will hate the spoil. Turnus acts like someone who thinks himself within the rule: he returns the body and grants the tomb. What he keeps for himself is what the narrator says he will come to hate.
The baldric ties together the three points of a given word. Evander entrusts his son to Aeneas in Book VIII, Pallas dies in Book X with the baldric on his shoulder, and in Book XII Aeneas, who in Book XI called the faith given to his father "my great faith," fulfills Evander's claim. He does not invoke the treaty, Latinus, or Rome: he names Pallas.
The ending admits two readings based on the same lines. By the first, there is justice: Turnus killed Pallas, uses the baldric as a trophy, and Aeneas fulfills what Evander demanded of him. By the second, the blow is the price of empire: the victor kills a surrendered man, who asked for his life in the name of an old father, moved by rage and not by law. The poem ends at the blow, without saying more.
The scene engraved on the baldric is a wedding night that ends in massacre, and the poem ends in a duel in which the victor gets a bride, Lavinia, and the vanquished gives her up before dying. The text does not comment on the coincidence, and there is no need to force it: it is enough to note that the object that decides the end bears, engraved, the reverse of what the end was supposed to be, a marriage that seals peace.