Could you be talking about visits to the Shadowlord, in the back of the rulebook?
There are also the "Rituals of the Chaos Gods" which are in the back of Town Cryer #14. They're not mutations, but instead offer a deity-specific spell for magisters and the ability to summon daemons of their patron god.
There are also the excellent "Corrupted Characters" mutations that Werekin wrote, which are posted on the Border Town Burning website.
Does any of those sound like it fits the bill?