Why a Fiend should be interested in becoming the skin of an Acolyte?

[Conradine]

The Ritual of Bonding is a blasphemy that was long ago eradicated from most arcane libraries, but a few barely legible copies—or at least references thereto—survive along with promises of great power. Spellcasters who happen upon such documents can choose to destroy or ignore the find, but the temptation has already occurred. Those who give in can eventually stumble upon the complete ritual, usually through extended contact with one or more summoned fiends that are all too eager to share their terrible knowledge.

Why? What they gain from that beside corrupting a single soul? And at the non negligible cost of being tied to a mortal for years, decades or even more.

[NigelWalmsley]

I would imagine fiends are quite enthusiastic about the suffering involved in playing a 5/10 caster PrC.

[Ramza00]

Isn't this a flavor question? I ask for it is thus up to the DM to figure out how this flavor takes place in their greater DM story.

Likewise the DM can reflavor the prestige class and say oh you are getting skin grafts but instead the outsider is merely giving you ectoplasm, or part of its skin that will regrow. Aka something like "SPAWN" of the 90s?

Alternative your DM can say you are making pacts with fey, and the fey is channeling the energy of the feywilds into your skingraft and so on.

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The flavor the books give you is the default flavor. But why this is, that is not answered by the books is ultimate up to your DM to incorporate the sample material why with his greater why. The interactions of layers of meaning.

[tyckspoon]

One of the central axioms of most default D&D fluff is that devils and demons are incredibly desperate for mortal agents, because the vast majority of them can't act outside of the Lower Planes without having a mortal ask them to in some way. And influencing mortals - trying to win the Prime Material over to Evil - is the whole point. Better to pretend to be a servant on the Prime where you can actively plot and try to advance the Evil Agenda than to be a servant to some other fiend in the Hells (this is also why evil Outsiders go along with Planar Binding spells, at least not when they're being forced into 1000-year 'you're my slave unless this one vanishingly unlikely thing happens' contracts by optimizers - you serve your binder more or less fairly, and your reward is *being on the Prime Material* with, usually, some free time and fairly loose restrictions on what you can do when you don't need to attend to your task.)

Spellcasters are well positioned to influence others, and one that is willing to become an Acolyte of the Skin in the first place is clearly at least morally flexible - not only do you get that soul, you can expect to be able to guide your bonded Acolyte into performing more and larger evil acts than they otherwise would. You're an eternal evil being; what's the cost of a few decades attached to an upstart mortal compared to what you might be able to gain from guiding them into working on your plans?

[InvisibleBison]

Maybe there are fiends who find this sort of thing to be fun.

[Ramza00]

Maybe there are fiends who find this sort of thing to be fun.

Are you describing "Venom" or "Carnage?"