Fuaran is a consumable item, which increases maximal mana of user by 100. It is required to have 200+ mana to use it.
Way of obtaining Edit
It simply drops from Golem with 10% chance.
Known issues: Edit
If you're playing on version, older than v7.0, then you may have an issue when item cannot be used.
Sadly, there is nothing you can do. Only way to fix it is to create a new character.