Mana is the concept of magical energy in InnWorld. It is inherent to all beings, but not everyone can spend it through magic. Mana can be released as a raw energy, ready for anyone to use it with Magic - that is the concept of free mana.
Each living thing can generate mana, but the amounts are wildly different. Toren is able to sense the amount of mana in persons, and he found that Ryoka produced four times the amount of raw mana that Erin produced, and Erin produced just enough to keep Toren alive. [Mages] like Pisces or Ceria were able to easily sustain 10 skeletons like Toren.
There are certain places that supernaturally accumulate free mana, especially dungeons. This attracts monsters to inhabit them, as they are drawn towards magical places. The mana of dungeons allows "regular" monsters to grow larger and more dangerous; and some monsters and undead can't even exist without constant mana supply like that of a dungeon.
Battlegrounds are another important place where free mana lingers; which is the reason that undead appear after a battle.
The skill [Magical Grounds] allows magic in The Wandering Inn to work more effortless, which means that the Inn also serves as a mana reservoir. It powers, among other things, Erin's Magical Door. Other magical artifacts on the inn's grounds that either consume or produce more ambient mana include the magical chessboard, the faerie flowers, the magically enlarged bee Apista, the [Field of Preservation], other Innkeeper skils of Erin, and various mages and druids living on the premises.
Magical energy can be stored in Mana Stones (similar to electrical batteries) or Magical Gems. Mages can load these stones with raw mana, which is considered an exhausting activity, but allows them to store their mana to later access it or to give it away. Such mana stones can be used to empower magical artifacts in the hands of non-mages.
Mana potions infuse a biological body with additional Mana. Spellcasters drink Mana potions to refresh their supply of magical energy.
It is considered unwise to drink a mana potion unless the imbiber has lowered their mana through magical activities. While the additional mana has the effect of energizing the person drinking that potion for a short time, this energy cannot be metabolized through other means than magical activity. The resulting effect is that of Mana Poisoning: Violent illness, fever, and uncontrollable bodily excretion. Too many or too potent Mana Potions can probably prove fatal.
Mana Potions don't mix well with alcohol. Drinking a mana potion when inebriated, triggers vomiting.