From Medivia Online Wiki

Intelligence is a stat only gotten through items, it increases your Spellpower.
It can be used by mages and druids only.
The amount of intelligence you get from a certain item is directly related to its Item Rank.

One point of intelligence increases your spellpower by 1.

Many equipment pieces and magic weapons provide Intelligence, often both as a Property and as a possible Attribute. Items like the Green Witches Potion can be used to increase your intelligence temporarily.