Luminarium

Luminarium was an in-game plot element and out-of-game mechanic used in ITM VI and ITM VII.

ITM VI
In ITM VI, those with special abilities were channeling a special and unknown force called Luminarium. The main characters discovered a particularly potent amount of it on a special island where they encountered an apparently benign Lovecraftian entity.

Luminarium was exhausted both by the use of abilities and by taking strikes. When Luminarium was exhausted, users would revert to their human state and be highly vulnerable.

ITM VII
ITM VII used the Luminarium system early on, but it was never specifically described in ITM VI. At some point, it was decided that Luminarium made some abilities such as healing work in strange ways and punished hybrid combatants who both burnt abilities and used frontline combat, so it was replaced by a more traditional HP and MP system.