Focus is a stat in Salt and Sanctuary. It is a white bar under the stamina bar, once this bar runs out you can not use any focus consuming abilities. Focus is primarily increased by leveling Willpower nodes in the skill tree. (Max Focus = 15 + 5 * Willpower). Focus primarily depletes from using spells, prayers, and incantations. Focus also depletes at a much smaller rate for any use of Stamina. This means that in addition all jumping, attacking, rolling, blocking, and magic abilities will also cause minor Focus consumption. (Lost Focus = Stamina Used / 400). It does not replenish automatically. Potions and Sanctuaries are the primary way to replenish focus. For each point of Focus missing the player accrues Fatigue on their Stamina bar. Focus is defaulted to half the value and size of stamina.
The consumption of Focus by any Magic is based on Equip % load.
Spells and Incantations are cheaper with lighter equipment burden, but Cleric Prayers are cheaper with a heavier load.
Spells use the formula: Cost = ListedCost * [(EquipPercent * 0.55) + 0.45] / 2
Enchantments use the formula: Cost = ListedCost * [(EquipPercent * 0.65) + 0.35] / 2
Prayers use the formula: Cost = ListedCost * ([(1 - EquipPercent) * 0.50] + 0.50) / 2
Note: Due to how the above formulas are constructed, all focus costs listed in the game are twice the maximum value.
Example: If you could cast a 100 cost Spell with 0% equipment load it would cost:
(0% * 0.55 + 0.45) * 100 => 22.5 focus
Then there's the tiny additional Fatigue caused by Stamina usage cost of: Stamina / 400 = Fatigue or 100 / 400 => 0.25.
This results in 22.5 + 0.25 => 22.75 and will be visually rounded to 22.8 in the game.
Saper Charm: Leeches focus from damage.
Conduit of Mind: Increases Focus, allows greater Fatigue.
Crystalmoat Ring: Halves effect of focus on Fatigue.
Burning Sky Ring: Reduce focus cost of Spells.
Relentless Ring: Reduces Fatigue from all Stamina costing actions. (Lost Focus = Stamina Used / 1000).