Did a search, don't think it's a repost. Not really sure how to link images or whatever.
To get the debuff, go to the location below and stand in the mist. No further action is required.
Location:
What it looks like on the ground:
Debuff lasts for 30 seconds on players and 10 seconds on pets. No quest required, easy to get full duration on pet by pre-casting dismiss while pet is running to location. Damage increase worked on me, haven't tested it on Stampede yet. Debuff remains on dismissed pet. There are probably better ones but this is pretty easy to get.