Nighthowler's Aspect is a Druid Offensive Aspect that can be found by completing the Forbidden City Dungeon as well as from random enemy drops. After clearing the Dungeon for the first time the Aspect will be available in your Codex of Power.
How to Obtain - Codex of Power - Forbidden City
Nighthowler's Aspect Mechanics
Legendary Aspects can change the way skills and builds work in significant ways.
Blood Howl's cooldown can be reduced by kills from other party members. The buff provided by Nighthowler's Aspect gives the Attack Speed and Critical Strike Chance to other Players but unfortunately not the 20 Spirit Generation. It also does not stack. You can't restore the Resource of any allies, including fellow Druids. When playing in groups, the buff cannot be refreshed until it expires. This means that your friend using a 10% Nighthowler's Aspect cannot overwrite your 5% Nighthowler's Aspect if you've used the skill first.
For further information, see our Build Guides, Legendary Aspects & Codex of Power