Hate

Hate is the calculation of how much damage you've dealt to an NPC.

The player having dealt the most damage to that NPC will be the target of his attacks and spells. At first, the NPC attacks are directed to the first one he saw, but as soon as he has been dealt damage by a player, the NPC may redirect his attack to the one at the top of his hate list. Unlike other MMOs, healing or buffing does not generate hate in DDO - only damage generates hate.

The ability to generate and retain hate of an enemy (whether you want the attention or not) is often known as "aggro".

Each NPC has its own hate list of each known enemy, paired with the degree to which it hates that character. The list is constantly sorted by hate, and whoever is at the top of the list is chased and attacked. Any character who drops below 1 HP is removed from the list.

Managing hate
When two players are attacking the same NPC, the one dealing the most damage will be the target of its attacks. However, the character dealing more damage is not always the one which would be best fit to withstand these attacks. The most obvious way to manage the monster's hate is obviously to stop swinging at him. However, there are other ways that have been implemented to manage it. The exact way those factors stack together is not known, although it is apparent they stack somehow (either additively or multiplicatively).

Enhancements
A few enhancements have been introduced to either increase the hate you do per damage dealt or to reduce the hate per damage dealt. Characters that are better suited to withstand attacks and/or spells may be attracted to those which increase the hate. Characters less suited for it, mostly for low hit points or low Armor Class reasons, may be attracted to those which reduces hate.

Increases hate

 * Warforged enhancements: Brute Fighting - Increases hate by 25/50/75% per melee hit, when active.
 * Half-Orc enhancements: Half-Orc Brutal Fighting - Increases hate by 25/50/75% per melee hit, when active.
 * Half-Orc enhancements: Half-Orc Brutal Spellcasting - Increases hate by 25/50/75% per melee hit, when active.
 * Defender of Siberys enhancements: Divine Righteousness - Increases hate by 200% per hit, temporarily. (1 minute duration per use)
 * Defender of Siberys enhancements: Defensive Stance, Improved Defensive Stance, and Superior Defensive stance give 50%, 100%, and 150% bonuses to threat generation, respectively.
 * Stalwart Defender enhancements: Threatening Countenance Increases hate by 20/40/60% per melee hit.
 * Stalwart Defender enhancements: Defensive Stance, Improved Defensive Stance, and Superior Defensive stance give 50%, 100%, and 150% bonuses to threat generation, respectively.
 * Monk stances: The four Mountain Stances increase threat by 50%, 100%, 150% and 200%, respectively.
 * Ki Shout: This Shintao Monk ability uses the Concentration skill for an Intimidate effect.

Reduces hate

 * Assassin enhancements: Shiv - Reduces the threat of all your attacks by 5/10/15%.
 * Thief Acrobat enhancements: Sly Flourish - Reduces the threat of all your attacks by 5/10/15%.
 * Thief Acrobat enhancements: Subtlety - Reduces the threat of all your melee attacks by 20%/30%/40%.
 * Archmage enhancements: Subtle Spellcasting - Your damaging spells generate 10%/20%/30% less hate than they normally would.

Increases hate

 * Vulkoor's Might and Greater Vulkoor's Might Set Bonus do 10% and 20% more Melee Threat, respectively.
 * Levik's Defender Set Bonus : 20% or 30%, depending on if the Set is Two-Piece, or Three-Piece.
 * Stalwart Defender Set Bonus does 15% extra melee threat.
 * Defender of Siberys Set Bonus does 15% extra melee threat.
 * Purple Dragon Knight Set Bonus does 10% extra threat.
 * Attacks from the Maul of Malice weapon do 20% more hate.
 * Various tiers of the Brigand's Cutlass benefit from 15% or 20% incite.
 * Dragontouched Crafting has incite 10%, 15%, and 20% on Eldritch, Tempest, and Sovereign Runes, respectively.
 * Anathema has a hidden effect of 25% increased spell-based threat.
 * Ring of Lies has a similar hidden effect as Anathema, but only 15%
 * Attacks from the Epic Hellstroke Great Axe do 20% more hate.
 * Fabricator's Bracers adds 20% additional melee threat.

Reduces hate

 * Tharne's Wrath 2-Piece Set Bonus is 20% reduced hate.
 * Vulkoor's Cunning and Greater Vulkoor's Cunning is 10% and 20% reduced hate, respectively.
 * Attacks from the Treason weapon do 20% less hate.
 * Wearing the Wretched Twilight reduces hate from spells and ranged attacks by 15%.

Skills
In addition, skills can temporarily override the hate sorting:
 * A successful Intimidate check on a monster will bring you to the top of the hate list.
 * A successful Diplomacy check will bring you to the bottom of the hate list.
 * A successful Bluff check will reduce hate generated and opens the target for sneak damage.

Special cases
Although most monsters use the hate system as described, there are some exceptions:
 * The Stormreaver clears his hate list each time he casts Reverse Gravity.
 * Flesh Golems are chaotic and have randomized hate. They do not consider the damage dealt; they simply attack anyone in sight and change the target very often.
 * Rust monsters prefer to attack Warforged.
 * Maruts have special hate for level 20 Rogues with the Cheat Death capstone (Which currently doesn't exist in game)
 * Bearded Devils, Horned Devils, and Orthons seem to have random hate when there is a lack of Intimidate, especially Named monsters.
 * Raid bosses have unique detection scripting to prevent indefinite hiding.
 * Raid bosses have unique detection scripting to prevent indefinite hiding.