Aggro

Aggro is the slang term for aggravation used to identify the amount of hate values generated from NPCs or monsters who gets attacked in YGGDRASIL.

Description
Monsters automatically launched an attack on the enemy attacker if the former were the first to get hit or targeted by the hostile player. Tank-type players are usually given the role in their party to control and offset a monster's aggro/hate, redirecting its attention towards them rather than the attackers.

When working alongside players in a party group, skilled aggro managers like tanks would not make mistakes such as allowing the magic casters in the backline to be attacked by enemies.

Mechanic
When monsters are attacked by various people, they would usually target the person who happens to deal the most damage or DPS to them. This could be expressed as some sort of numeric value, called aggro or hate. Causing more damage, healing HP, buffing, debuffing, or using taunting-type skills would increase the monster's aggro over time. When a player get hit by a monster, it would somewhat resolve the matter to some extent as that monster’s aggro of the player would decreasingly go down during the process. Since hate values tend to fluctuate often during a battle, tanks had to pay very close attention to the monster's hate values, or else the backliners will take a hit if left unguarded.

In other words, to combat this aggro problem, the person must be skilled at hate management, which was a difficult task to successfully do because hate was classified as a hidden value. There was virtually no way to know how much hate is built up by any given action produced without personal experience involved. A monster's aggro could also be beyond the tank’s ability to manage if their party member's aggressive actions tend to go wrong for the latter. For instance, doing too much damage made aggro management difficult, one of the causes that can give the tank a hard time controlling the flow of the battle. In order not to build too much hate, the attackers must sometimes use a weak spell instead.

For that reason, using potent spells or skills to destroy a monster always carried the heavy risk of aggroing all the other nearby monsters in close proximity. If the player is part of the same race as the monster, then all likelihood, there was a high chance for the monster to not make a move on their attacker. Any aggro that had been generated from attacking the monster can all go away with its death by killing it first. Even after resurrecting the slain monster, the latter's hate value will completely be reset back to zero, leaving it unable to demonstrate hostile or offensive actions towards the person who once killed them.

Trivia

 * For players like Bukubukuchagama with the role of tanks in a party, it requires great management skills to control a monster's hate and hence, it was considered a hard job. Being able to do such a thing in the data-heavy YGGDRASIL was a feat that required long hours of battles and practice.
 * It was said that a tank like Albedo could redirect aggro between the summoned monsters in a way where they would have to kill each other as a result.
 * In order to manage monsters who are either part of the golem or undead races, with immunity to mental manipulation, tanks had to utilize certain specialized hate control skills.
 * Death Knights possessed an ability which allows it to draw away enemy attacks of those disturbed by its appearance. They take on the monster's hate/aggression directly as a temporary shield for the summoner or their party members.
 * Bukubukuchagama simultaneously used [Shield Attack], [Shield Stun], and [Mega Impact] as a Hate Combo.
 * According to Ainz, the mercenary NPCs hired by players had poor management of aggro with enemy monsters. He presumed it was made this way intentionally by the devs, suggesting that they want every player to come together as a party and worked on handling aggro.
 * Ainz Ooal Gown noted that it was very hard to manage the hate of Evil Lord Wrath than other Evil Lords because the monster can built aggro more easily. He had heard tanks say that the most aggravating thing about dealing with multiple Evil Lords at once was how to keep the Evil Lord Wrath from going off-target. Additionally, it had the special ability to do more damage and gain more defense the higher its hate value became.