Mana (マナ Mana) or otherwise called MP is the term to represent the magical power possessed by Magic Casters who need it to use Tier Spells in both YGGDRASIL and the New World. It served as fuels for the successful casting of both Runecraft and Tier Magic respectively. Tier Magic itself is powered by mana.
Mana was said to be the main source for practitioners to rely on while using the spells of Tier Magic. When Gazef Stronoff tear apart angels, the blood that pour out from these monsters' wounds are said to be their "mana." This suggests that the essence of mana itself was what composed the life force of monsters in general. A magic caster's mana is often conserved or reserved carefully and later used whenever they are forced under certain situations. Taking into consideration the magic caster's mana capacity, casting appropriate spells requires perfect timing for them depending on how much they possessed.
During a war of attrition against warriors, recklessly using up one's mana is a bad approach for drawn-out fights with them in the short-term. The consumption of mana can end up being wasted if their spell casting was disrupted by outside forces. For those who specialized as a full-blown magic caster, this kind of disruption would have been nothing and that using up mana will not be left in vain as it hardly affected them. However, if one had instead trained to be both a warrior and magic caster, even a minor disruption was enough to easily break their mental concentration while attempting to cast. With interference, this would cause magic casters to wastefully deplete one's mana, failing to properly complete the casting of their spells.
While so, magic casters release their mana for different purposes based on the unique role and repertoire of spells they have in when working in a group. Nonetheless, the magic casters who had run out of mana will resort to using Magic Scroll or Wands to cast their spells instead. In a way, there were all sorts of magical items with ample mana stored inside them. According to Ainz Ooal Gown, his YGGDRASIL short-sword is imbued with about as much data it has to the mana contained in Gazef Stronoff's Razor Edge weapon. This suggests that data and mana are exactly one and the same thing.
On the other hand, Pluton's Magic Shortsword that was once owned by Ainz noted how little mana there was inside it. What makes Gazef's weapon different from YGGDRASIL was Fluder Paradyne's speculated notion that it may perhaps be spontaneously created by the natural absorption of mana. Additionally, there was also a dwarven-made weapon with mana flowing through it found in the Dwarf Kingdom's treasury. Along the way, Runes with Runecraft origins are characters imbued with the use of mana. All in all, one can judge how powerful those items were based on the amount of mana they contained.
Mana is connected to the tier spells used and casted by magic casters. It is part of the user's magical power residing in their body. Without mana, magic casters can't cast their tier spells. They will feel exhausted as their breathing becomes ragged after consuming almost all their mana. This was considered as the symptoms of mana exhaustion. The intense mana toll could result in making the magic caster become immobile. Eventually, one would lose consciousness from overusing mana beyond their set limit. In order to quickly regain the expended mana they have used up when casting their spells, this requires the person taking a rest or conduct a meditation for as long as need be until it is fully restored. The same could be said with restoring one's mana through going to sleep too. However, sustaining the transformation spell such as [Perfect Warrior] would both lower mana and mana recovery rates to zero which can counteract that process. Though one could dispel the transformation if there was an emergency and use magic, the magic caster would be starting out from a depleted state of mana. Within the New World, depending on the magic caster's mana recovery rate and mana, it can possibly take more or less than a day's worth of time to fully recover back all of one's depleted mana reserves. In the case of Kelart Custodio or Ainz, it requires an entire day for a complete mana recovery. Alternatively, it is possible for a magic caster to transfer their mana over to another person if need be.
Monsters summoned through the use of Tier Magic had its bodies created from their summoners’ mana. A defensive barrier created from a spell is also composed of pure mana. Under YGGDRASIL standard, the holder of the Blood Drinker class has the [Blood Pool] skill which lets them utilized blood as their secondary source of mana. This was done so that one could used it to augment spells without consuming extra mana from their actual source. In order to harm a high-level being as Shalltear Bloodfallen, a weapon had to be made of sufficient mana for it to work. There are spells like [Mana Essence] to identify the foe's aura of radiating mana or even [False Data: Mana] which can counter it, deceiving the opposition with a fake amount. At the same time, there exists a necromancer spell which lets the caster create a permanent undead to stay in the New World as long as sufficient mana and corpse is provided. Maintaining skills or other spells like, [Invisibility], [Fly], or [Control Amnesia] can deplete one's mana reserve over the course of time throughout its duration until the caster runs out of mana for it to finally deactivate. It happens to interfere with the magic caster's mana recovery rate. In order to defend oneself from a powerful spell, the magic caster needed to pour the right amount of mana equally comparable to it in strength while using a buff spell that might help the user's resistance and so on. Consuming mana when using a buff spell leads to the caster's body emanating a brief glowing aura before disappearing. Whereas objects made of mana derived from the casting of attack or defense type spells.
Ranging from the 1st tier spells to the 10th tier spells, each one are varied by the amount of mana needed to use it during the spell-casting process. For starters, the higher the tier of a spell is, the higher the cost of mana that have to be required and utilized for casting it while as the lower tiers need to have a lesser amount of mana to do so. It functions this way systematically unless such tier spells are applied with Metamagic Enhancements like Over Magic. After the magic caster exhausted a large amount of their mana, this enhancement enabled the use of higher-tier magic that was not usually accessible for them. In YGGDRASIL standard, the normal time it took to completely recover one's MP back to full was six hours. Basically, if one end up using only used the highest tier, they would soon run out of mana to go on casting. The base MP was the player character's level times ten. When it comes to the nine worlds from YGGDRASIL, no convenient potions are known to exist in the game for dealing with the restoration of the magic caster's mana. The only way to recover it was by the passing of time. Most solo magic casters save up their mana reserves with means like equipping either wands, staffs, or scrolls. Furthermore, it was said that there exists a useful ability for equipment which allows for the reduction in mana consumption.
Moreover, one can even channeled their mana into their magic weapon to activate its ability. However, as a result, there are some magic items that drain a certain amount of mana from its user. For instance, consider a magic necklace that consumed a specific amount of the holder's mana in order to cast its spell. While one could use the item's spell indefinitely as long as one possessed sufficient mana, it consumed more of the user's magical power than casting the spell directly without it. Uniquely, a scientific ranged weapon like a spell gun was also capable of firing bullet-like bolts through the user's mana reserves. There were several theories deducing why so many Adventurers were stronger than an average human being. One of such theories pointed out there was that they happen to be absorbing mana from Monsters while fighting it.  Although that speculation has yet to be proven true, the same could not be said with various monsters who grew stronger with mana. In this case, Elder Liches were undead entities who continue to absorb a great deal of mana over a long period of time will eventually transcend into their evolved state of what's now known as Night Liches. Night Liches such as Kunivela are capable of sensing their foe's mana as long as it is not concealed by unknown means like magic items.
- According to Shalltear, Ainz Ooal Gown had about one and a half times the MP (mana) of her own power. She believes that there was probably nobody else but Ainz alone in the Great Tomb of Nazarick who had an excessive amount of mana like that.
- Fluder Paradyne or Arche Eeb Rile Furt have a talent that allows them to perceive a magic caster's aura or presumably their mana reserves based on the extent of how high they can use a certain tier.
- Evileye has a spell known as [Translocation Damage] that turned the physical damage she received from her foe into mana loss.
- Unlike magic casters, warriors have very low mana reserves than their counterpart. However, hybrid magic warriors like Shalltear possessed more mana over a pure warrior but still falls short compared to a pure magic caster.
- Based on the conditions of using the [Perfect Warrior] spell, YGGDRASIL allowed magic casters to bolster their stats at a heavy cost of their own mana, mana recovery rate depleting to zero as well as the fact they can't use any other spells but namely a few permitted.
- A person's mana is normally invisible to the naked eye without means like an item, a spell, a talent, or even an ability to let the latter perceived it. Not just that, but a person can also go out of their way to hide their mana reserves with similar means to ward others off from being able to detect or perceive it.