Pathfinder 1E Who as the big bad?

Recently started a PF campaign set in Osirion that will feature a " be careful what you wish for" theme with a nod to Berserk.

The Big Bad (must be someone/something with great magical powers) is discreetly handing out Behelit-like items that twist and change people into monsters according to what they secretly truly desire (besides Berserk, most of the inspiration from this comes from the Soul Sacrifice game and the backstories of the monsters you fight). Most of the events in the campaign will result from the actions of these transformed beings.

Now, who could be behind this? Is this something Ahriman would do? Or would some other entity be a better fit?

You may want to look at the later parts of the Legacy of Fire adventure path.