I can't remember a Prestige Class...

[Conradine]

I vaguely remember a prestige class which involved a demon cultist gaining levels of Sorcerer thanks to instruction from his demonic master.
It's not the one from Fiend Folio ( that was about clerical magic ), I'm sure it was about sorcerer magic.

I know it's very little but does it rings any bell?

[Doctor Despair]

Was it Demonbinder from DotU?

[Conradine]

No, it wasn't warlock, it was Sorcerer. It impressed me because it wasn't generic spellcasting but specifically Sorcerer. A demon who makes a Sorcerer from a cultist with no innate spellcasting abilities, that hit me.

[thorr-kan]

Nar Demonbinder, from Unapproachable East?

It does not advance sorcerer casting, but its casting is Spells Known/Spells per Day. They start at 4th level.

[Thurbane]

Was this 3.5, or PF?

In 3.5, no class that specifically does this that I am aware of.

Along with Nar Demonbinder, Demonologist (BoVD) has it's own spontaneous casting progression.

The closest to what you are describing might be Dragon Devotee (RotD), which grants two virtual Sorcerer levels over 5 levels. It would be possible to re-fluff this as a Demonic PrC, rather than Draconic - though you'd have to tweak some of the abilities to be more appropriate to Demons.

[Conradine]

Solved, it was the Nar Demonbinder. Thanks everyone for the help!

[ShurikVch]

Solved, it was the Nar Demonbinder. Thanks everyone for the help!
Gee, no wonder nobody could remember it - you said:

A demon who makes a Sorcerer from a cultist with no innate spellcasting abilities
while Nar Demonbinder (http://archive.wizards.com/default.asp?x=dnd/ex/20030502a&page=2) required

Spellcasting: Ability to cast 4th-level arcane or divine spells, knowledge of at least one summon monster spell.


I know you said it's solved, but Maho-Tsukai (Oriental Adventures) may fit the OP a bit better
Also, once I seen somewhere the full Sorcerer spellcasting was added to Mountebank (Dragon Compendium)