Trying to understand the herald skill gems

If I put on a support skill gen for say critical strike multiplier does it make every shot I use with tornado shot when it crits multiple or should I put it on the tornado shot itself and what gems work with the heralds better the one who use chance to ignite or shock or freeze is poison any good trying to fix my skills to do more damage and it's difficult to understand what I'm do and what gems I should be using

Heralds skills are buffs, not auras, which grant your character two kind of bonus.

The first bonus is applied directly on the character.
* For Ice and Thunder, it adds flat cold and lightning damage respectively to attack and spell.
* For Ash, it gives the character more spell Fire damage and more burning damage
* For Purity, it gives the character more spell damage, both for attacks and spells,
* For Agony, it gives the character increased chance to poison with hits, if those hits are capable of deliver poison to begin with (usually, this means they must either have a physical or a chaos damage)

The second bonus is a secondary effect that benefits the character when they do something:
* For Ice, it release an ice nova from enemies killed while chilled
* For Thunder, it gives the player a temporary buff that strikes nearby enemies with a lightning bolt that cannot Shock whenever they kill a Shocked enemy
* For Fire, it release a fire nova from any enemy killed, whose base damage is equal to the Overkill damage and it's dealt as Burning damage (therefore it is increased by anything that boosts Burning damage specifically)
* For Purity, it has a chance to summon Sentinels of Purity when the character kills an enemy or hits a rare or unique monster
* For Agony, it will add 1 Virulence Stack to the player for every 1 poison stack they add to the enemies or to themselves. If the character has at least 1 Virulence Stack, an Agony Crawler will be summoned, and its attack will be boosted by the player's Virulence

Except for Purity and Agony, whose effect is a minion summon, all the other herald's secondary effect is to be considered as Secondary Damage. It is neither a Spell or an Attack Damage, and cannot be reflected.
Also, those secondary damages have a natural 0% Chance to Crit, therefore they cannot crit unless you support those skills by adding a base Crit Chance to your character (for example, Assassins, or by supporting the Herald with a Increased Critical Strike support)

When you support an Herald with a support gem that provides bonus to damage, those bonus are given to the secondary effect of the Herald, and not to the buff effect on the character. For example, supporting HoT with CritMultiplier will not give your character extra critical strike multiplier, but will instead boost the critical strike multiplier of the Thunder Bolt the herald releases when you kill a shocked enemy - which means it's wasted since it has 0% natural crit chance. Similiarly, supporting a HoI with Added Cold Damage will increase the damage of the novas, but not the added cold damage to the character's attacks and spells

Most of the support gems you can link to the Herald will usually only care about the secondary effect of the Herald. Nothing can boost the damage those Heralds give you (in case of Ash, Ice and Thunder), only "Increased Skills Effect" (not Aura effect since they are not auras)

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Maxtrux wrote:

* For Ice, it release an ice nova from enemies killed while chilled

While frozen, while chilled would be pretty OP.

Thank you Torguemada. Had a slip there.