Demon Fury is a unique feature exclusive to the Demon Slayer class. It determines how many times a Demon Slayer can use their skills in conjunction with their HP. Every Demon Slayer have a base number of 10 Demon Fury, which remain constant and can be increased when given a new Force Shield.
Demon Fury and Mana
Both Mana and Demon Fury shares a good role in players skills, but they have very noticeable differences, as well as some benefits. Unlike Mana, Demon Fury requires no potion or buff to replenish it and it requires a first job skill to recharge it, so potion spending is cut in half. Demon Fury is far more convenient than Mana in the case of fighting a hard foe, you will not have to worry more about recharging.
However, Demon Fury cannot recharge on its own unless you use Demon Slash on enemies or activating a third job skill that automatically recovers Demon Fury. As said in the first passage, Demon Fury has a base number of 10 DF and it cannot be increased with any skill unless you advance throughout your jobs. There is no potions or buffs that can replenish Demon Fury and there is no scroll or type of potential relating to Demon Fury. And even though you use no Mana at all, Demon Slayers are not exempt to 1/1 attacks, or skills that can reduce your HP/MP to 1; your Demon Fury will also lower to 1 as well.