Rolling 3 to cast Arcane Bolt deals 3 damage for 3 Mana. Rolling 15 deals 15 damage but costs 15 Mana! You must have at least 1 Mana to cast a spell. However if you cast, and the Effort exceeds your current Mana, that’s fine. The spell still works, with the Effort you rolled. But your Mana is reduced to zero.