Ive read before on tipit, that there is an altar accesible from doing a quest, that gives + 15 hp , or like a 15% hp bonus when you recharge prayer at it... Is it true? what quest is it? and does it work in the duel arena, no matter what rules are set?

It is the altar in a desert town, Nardah i think was it's name. It becomes accessible after you've done the Spirits of Elid quest.