Like the other Demonic Fountains, Fiendish Fortitude features 4 Tier Upgrades with the base Tier being free and every subsequent Tier having an increased cost: 2500 Essences for Tier 2, 5000 for Tier 3, and 7500 for Tier 4. When downed, the player will keep the Perk but will lose one Tier.
Prior to the 1.09 Title Update on December 16th, 2021, Fiendish Fortitude increased the health of the players by 25 for each tier, up to 200. Following the update, it now increases the health by 50 for each tier, up to 300.
- Der Anfang - At the doorway to the Office.
- Terra Maledicta - At The Spike.
- Shi No Numa - The Balcony Stairs at the main hut.