Commander of Darkness
Commander of Darkness is a Notable Ascendancy passive skill for the Necromancer that grants increased damage, attack speed, and cast speed to the user and nearby allies for each non-curse auras cast by user. It also grants elemental resistance to you and your allies if they are being affected by your aura.
Each Aura the player as active grants 3% Attack and Cast Speed PER aura to the player and Allies. The 30% damage bonus is not related to the amount of auras active. It's a 30% flat increase whether the player has 1 or more auras active. Same with the Resistance bonus. It's +20% to all Elemental Resistances no matter how many auras are active...as long as one is active.
*Prior to 3.0 (Sorry, I don't know where deleted content should go)*
The Damage and Speed effects of Commander of Darkness are classified as an aura and are added onto the effects of whatever non-curse auras the player has cast. Thus, the effects that are granted can be further enhanced by the Necromancer's own Aura Effect stat, with the resulting effect always being rounded down to the nearest integer. This is done on a per aura basis.
For example, a Necromancer with six auras active and 50% Increased Aura Effect will grant 15% Increased Damage, and 7% Attack and Cast Speed per aura (not 7.5%), granting a total of 90% Increased Damage and 42% Increased Attack and Cast Speed. Note the speed is not 45%, as one might initially expect; this is the result of the rounding down per aura.
The Elemental Resistance granted by Commander of Darkness is applied only once, and is not classified as an aura, so it cannot be further enhanced.