I was wondering if there ways to change a character's alignment during the campaign, through either actions or quests or items.

Basically, I would like to make a paladin eligible to multiclass into Cleric of Mask: either true neutral, or any evil. I tried to transform him into a fallen paladin, but killing npcs doesn't seem to work (or I'm doing it wrong).

First, I'm pretty sure that's impossible.

Second, if you dualclass your paladin into anything but the class her order sanctions, you will lose all your paladin abilities. Only the Paladins of Ilmater can dualclass into clerics, but ONLY clerics dedicated to Ilmater (i.e. the Painbearers of Ilmater).

Third, if you were able to change your alignment, in principle you should be able to lose all your paladin abilities anyway if you become anything but lawful good.

My plan for using the Holy Avenger has been foiled.