YGGDRASIL

YGGDRASIL (ユグドラシル Yugudorashiru) is a fictional online game in OVERLORD series. It is a DMMO-RPG (Dive Massively Multiplayer Online Role Playing Game) developed in Japan, serviced from 2126 AD to 2138 AD.

Introduction
YGGDRASIL needs an exclusive console to play, which records logs for a week. Though the game is a full virtual reality one, in accordance to physical laws, it does not provide a sense of taste and smell, and has a limited sense of touch. Due to the lack of such technology when the game was developed, facial expressions and artificial intelligence are not supported.

The most distinctive feature of YGGDRASIL is its flexible system. Players can select numerous races and jobs and various tools allowing for customization. It seems that the customization system is really far-reaching, even extending to basic programming.

World
As it can be guessed from the title, YGGDRASIL mainly refers to Norse mythology, and has nine worlds based on it : Asgard, Álfheimr, Vanaheimr, Niðavellir, Midgard, Jötunheimr, Niflheim, Helheim and Múspellsheimr. Each world is a leaf of Yggdrasil. Other leaves were eaten by Navagraha the World Eater, or shed and transformed into World Class Items.

Level
The maximum level a player can reach is 100. When a player character dies, it loses 5 levels and drops one's equipment. Death of character with level lower than 5 just makes one back to level 1. This penalty can be relaxed by using certain revival skills or cash items. When a player revives by skill, it can select the location he or she desires. NPCs belonging to players or guilds can revive instantly by paying gold.

Races
Races of YGGDRASIL can be roughly categorized into three taxa. There are more than 700 races, including racial class that needs certain level of low rank racial class to learn. Some items can change races permanently. In the game, humans and demi-humans, who had limited lifespan, would grow and age normally. In contrast, heteromorphic races with unlimited lifespan would stop growing after a certain stage.
 * Human races include human, elves, dwarves, etc. Human races are weakest in basic status and cannot have racial level. Instead they can learn more classes which provides high status or powerful skills more easily, giving them high potential. Adding familiar appearance to these advantages, human races were most popular in YGGDRASIL.
 * Demi-Human races include orcs, orgres, lizardmen, etc. Demi-Human races can get more basic status and have racial level. Demi-Human races are similar to the average between Human races and Heteromorphic races.
 * Heteromorphic races include slimes, vampires, devils, etc. Heteromorphic races own most powerful basic status, which can be even more strengthened by increasing their racial level. High-rank races also provide special ability similar to that of monster's. Yet Heteromorphic races were very unpopular, as the racial level limits job level one can learn, and PK on Heteromorphic races doesn't give any penalty to the player who killed Heteromorphic race player.

Classes
More than 2,000 classes exist in YGGDRASIL. Basically 15 is the highest level reachable in a single class, yet some high-rank classes seems to have lower level limits. Thus a 100 level player owns at least 7 classes. If one ignores efficiency, it is also possible to get a hundred 1 level classes. Of course there are some 'optimized' class builds discovered by players.

Guild
A guild can conquer a dungeon and use the place as it's headquarters. In this case, the guild is granted NPCs to protect it's base. There are tree categories of these NPCs. There were guilds that also conquered and used a city as their base in YGGDRASIL. It was a common occurrence for different guilds to lay siege to a city. Ainz stated that setting up a base in a city would make some of the areas hard to defend against.
 * Spawn NPCs pop-up automatically from the dungeons. They don't need gold to summon or maintain, yet they are under 30-level and do not have enough power to protect the stronghold.
 * Mercenary NPCs can be summoned by spending guild finances.
 * Custom NPCs can be customized freely, with their power level depending of the level of the guild stronghold. The lowest-ranked guild headquarters provides 700 level to be shared between NPCs. This can be increased by upgrading to an higher-ranked dungeon, using cash, etc. Customization tools includes not only level, equipment, and appearance but also AI. Thus custom NPCs can be made and used as powerful guardians. Of course a guild is totally free to customize it's NPCs just for decoration purpose.

Magic
More than 6,000 different spells exist in YGGDRASIL. A 100 level spell caster usually knows around 300 of them (3 x caster level in the Web Novel.). Even basic functions such as message needs spells to use, so most of the spell casters have trouble managing all the spell they desire to have.

Magic is divided into eleven ranks: Rank 1 to 10 and Overrank Magic. Overrank magic has miraculous power even compared to 10th rank Magic, but suffers from huge casting and cool down time, preventing too frequent use. Casting over rank magic decreases the caster's defense, and receiving a certain amount of damage cancels the casting.

Items
Items are classified according to the following rank system: Items drop as a data crystal, and players can produce items freely in the capacity of crystals, including options to customize their appearance and name. Most players named their equipment in reference to legends and mythologies, in order to make them easier to sell.
 * Lesser → Low → Minor → Medium → Major → Greater → High → Highest → Legacy → Relic → Legendary → Divine
 * Artifacts are not related to specific ranks. It just means they are items with fixed data. So artifacts with low rank can exist.

World Class Items are items whose power overrides the limit of other items. Many of them can ignore or even change the game system of YGGDRASIL.

Monsters
The most powerful monsters in YGGDRASIL are called World Class Enemy. Even Legion (6 parties with 6 players each) can have hard time fighting against them. Some of them are related to World Class Items. There were about 32 level-breaking bosses and are listed below:
 * Devourer of the Nine Worlds.
 * Eight Dragons
 * Seven Demon Kings of Sin
 * Ten Great Angels of the Tree of Life
 * God of the 6th Day
 * Five Rainbow Buddhas

YGGDRASIL and the New World
Once every 100 years, players of YGGDRASIL are teleported without reason into the new world. It seems that the first teleportation had occurred 600 years ago. With the knowledge and exceptional power of YGGDRASIL, the players usually shake the very foundations of the new world, making them legendary godlike figures.

Ainz Ooal Gown is currently the only known player to have been teleported to the New World when he was expected to be forcefully logged-out of the game in the end of YGGDRASIL and later decided to make an everlasting name for himself in the New World he is in now.

Trivia

 * In Norse mythology, YGGDRASIL is the World Tree, a great ash tree located at the center of the universe and joining the Nine Worlds of Norse cosmology.