And since everybody loves my random D&D recollections:
Generally not. There’ll be more scope for that with the hostile outsiders in XOR, as I’ve moved away a bit from the implacable humanity/reality-destroying forces of my earlier work (though neither the Halassurqs nor the Unquiet Dead will be easily bargained with).
In my old D&D campaign, both the sea elves and the sand elves had a powerful renegade who was sympathetic to humanity; the party made the most of those relationships in averting the threat from the otherwise implacably human-hostile elves.
The party did manage to temporarily form a detente with three ghastly archmages who nearly destroyed the empire 300 years ago – Syraxus the Damned, Nul-Gashar the Nightsummoner, and Z’kaatra of the Blood Raven, that necromancer son of Death I mentioned earlier. The detente lasted long enough for them to collaborate on a solution for the genetic mind control that the sand elves exerted over human mages. After the all-out war resumed, the party managed to kill Nul-Gashar in a grand battle over the Holy City of Deshiao when one of the party’s undead members used his power to travel through shadows to leap across half the city (by rolling with a 3-in-100 chance of success), and disrupt the Nightsummoner’s spell so that he was eaten by his own extraplanar portal. That was a fine game. 
Deals with Death were integral to the plot of the game; by contrast to most D&D worlds, in this one there was no Raise Dead spell, just Summon Death (and cut the best deal you can, if he’s interested). But after one of the party members refused to accept the terms of Death’s bargain – handing over her Deathborn child, which had been touched by Death in the realm of True Forms and shared some of his power – they became targets rather than beneficiaries of Death’s deals. We were still probably many game-years and a lot of gaming sessions out from the point where little Amaranth would defeat her “father” and become the new Death; but she had already got his sword.
With the Yrchhu, the Horror of the Many, there is no negotiation. Any more than there is with Xaos-storms.