Well, im not 100% sure about the source of the buff, but i think its from the quest when you touch a stone to meet the "Wilderking", he talks to you and he sends you to touch another stone to fight an "old enemy" of you, i used the persuade option for asking for his help in the fight, and i think he buffed me with that.
"Spirit Armor", +5% armor
But the buff have no timer, i finished the quest and many others, i loged out and a day passed, and i still have the buff, is just there like the Mundus Stone Buff, i dont think thats inteded, so im reporting it, even if that its a "good bug"