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 onto someone if they were the first to get hit. Tank-type players are usually given the role in their party to control and offset a monster's hate, redirecting its attention towards them rather than the attackers.

Mechanic
When monsters are under 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 hate. Causing more damage, healing HP, buffing, debuffing or using taunting-type skills would increase the monster's hate over time. When a player get hit by a monster, it would somewhat resolve the matter to some extent as that monster’s hate 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 for.

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 hate could also be beyond the tanks’s ability to manage if their party member's aggressive actions tend to go wrongly for the latter. For instance, doing too much damage made hate 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.

Trivia

 * For players like Bukubukuchagama with the role as tanks in a party, it requires great management skills to control a monster's hate and hence, it was consider 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.
 * In order to manage monsters who either golems or the undead, 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 it's 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.