The calling of the Covenant is to keep the Spark of hope alive, in a deathless land that erodes the soul over time like rivers through cliffs. Few things can diminish one's hope like watching DPS or Heals get downed by a boss heavy they decided not to dodge. Thus at the apex of righteousness is the Tank—dauntless bastion, ever-protecting the squishy party from certain doom. Learn and reflect upon these Tanking Tenets if you wish to embrace this noble duty.
Threat is the mechanic which determines whom a given enemy mob will target; this enemy behavior will hereafter be referred to as "aggro." Each enemy mob has a hidden "Threat table," which is a list that orders the players from highest to lowest Threat. The mob will aggro whomever is at the top of the Threat table. Players raise their Threat value in two ways: dealing damage to the mob, or using healing ablities in its vicinity. When you have the mob's aggro, their health bar will have a red hue around it, as seen in the image below.
It is the Tank's duty to keep aggro from the rest of the party by remaining at the top of each enemy's Threat table by utilizing Carnelian weapon gems, the Hated perk, and Taunt abilities.
When slotted into weapons, Carnelians increase the amount of Threat you generate when dealing damage or using healing abilities. Carnelians also cause certain Weapon abilities to apply Taunts, expounded upon in Tenet 4. You must have Carnelians in your weapons if you are tanking; Expedition queues even require it. Here are the Taunting perks and their Threat generation bonuses associated with each Carnelian grade:
The Hated perk on a weapon will further increase threat generation when using that weapon. Between GS 400 to 725, you will generate 157% - 245% more Threat.
While this extra Threat generation isn't necessarily needed if you're using Cut Pristine Carnelians, it can be useful for higher-Constitution builds with less damage, as covered in the Build Tenet.
Beyond Threat generation, there several armor perks that are helpful for Tanks:
The most advanced technique of the Tank is Taunt ability management. With a Carnelian in your weapon, certain abilities will apply Taunted—a status effect that artificially puts the Tank at the top of the Threat table of affected mobs, regardless of how much Threat they're actually generating. During this time, the Tank must deal sufficient damage to the mob to hold aggro once the Taunt expires. Some Taunt abilities only affect one enemy, while several apply to all enemies in an area of effect.
The Taunted status effect lasts ???. It is vital that the Tank spaces out the use of their Taunt abilities, rather than expending them all at once. New mob waves may appear suddenly, necessitating a quick aggro grab before they dogpile on your party members. So long as the Tank is attacking mobs or bosses while they hold aggro, Taunts should not be necessary; however, sometimes the ability is worth using for its standard effects, such as granting damage mitgation through Fortify.
Every weapon has at least one Taunt-viable ability in each of their two ability trees; some have even more. In lower-difficulty Expeditions, any weapon can theoretically tank, but optimal weapon choices start narrowing in more difficult dungeons and Mutation levels. Weapon choice is elaborated upon further in the Builds Tenet. Here are all the Taunt abilities for each weapon: