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
It simply drops from Golem with 10% chance.
Knows issues:
If you played with version, older than v7.0, then you may have an issue while item cannot be used.
Sadly, there is nothing you can do. Only way to use it is to create a new character.