Livre des saintes paroles et des bons faiz de nostre saint roy Looÿs (1305-1309)

Written as a biography of Louis IX during the Seventh Crusade, Jean de Joinville's Life of Saint Louis documents the culture surrounding the Crusade. Using information from André Longjumeau, Jean narrates the destruction of Prester John by the Mongols (Silverberg, 96-111). The story is similar to those recounted by the other missionaries deployed to Khanbaliq in the mid-thirteenth century.  

Vitale (p. 16) summarizes Joinville’s contribution on the legend:

Joinville, who accompanied Louis IX on a Crusade in 1248-49, writes in his Chronicle some fifty years later that Andrew of Longjumeau and his brother, both Dominican priests who spoke Arabic, had been sent as emissaries to the Mongols. Andrew reported to Joinville that the Tartars (Mongols) who had been subject to Prester John had risen up and defeated him.

Joinville's describes the death of Prester John (qtd. in Brewer, pp. 193-4):

After [a Mongol prince] had arranged and ordered these things, he said to them: 'My Lords, the strongest of our enemies is Prester John. And I command that tomorrow you all be ready to move against him, and if it happens that he defeats us (may God spare us from this!), may everyone do the best he can. And if we defeat him, command that the occasion extend for three days and three nights, and that no one be so bold as to put his hand to spoils, but rather kill people, because after we have secured the victory, I will distribute the spoils among you so well and so fairly that everyone will be satisfied'. To this they all agreed. The next day they moved against their enemies, and they were victorious, as God had wished. They killed all those whom they found defending with arms, and they did not kill any whom they found in religious garb, the priests and the other religious men. The other people of the land of Prester John who were not in the conflict all placed themselves in lheir power.

See also  Silverberg, pp. 96-111. 
Read an English translation of Joinville here. 
To read Joinville in the original French see Wailly’s edition.