The Secret Treasure of Itzpalt is a one-use item. When used, it grants the racial feat "[Race]Elemental spells you cast are empowered" which boosts the effects of each elemental spell (fire, cold and lightning) that the user casts. It is given as the second reward for worshiping Itzpalt of Element.
In Elona+, the feat grants +25% to the dice sides (e.g. the 143 of "25d143") of elemental spells. It's unknown if this is true in vanilla Elona however.