Blizzard has unlocked the Heroes of the Storm cross promotion event For Azeroth!. You can receive a Primal Flamesaber Mount in World of Warcraft just by playing HotS.
The event runs until March 13 and has some quite simple rules:
Play 15 Heroes of the Storm games with a Warcraft hero while teamed up with a Battle.net friend.
The type of game (Ranked, Custom, …) you play does not matter.
Gather your friends together and play any Warcraft Hero in Heroes of the Storm as a team and you can earn the new Primal Flamesaber Mount in World of Warcraft.
Beginning February 14 through March 13, a new quest called For Azeroth! will be available in Heroes of the Storm. Play 15 Heroes of the Storm matches with a friend from your Battle.net® friends list, and you’ll be burning a new trail through Azeroth in style.
When: February 14 – March 13, 2017
Where: Play Heroes of the Storm Free
What: Play 15 matches of Heroes of the Storm using a Warcraft Hero while teamed up with a friend from your Battle.net friends list.
Don’t have a Warcraft hero in Heroes of the Storm already? Don’t worry—each week a new set of heroes will become available to play for free. See the hero rotation schedule and begin planning your path to victory within the Nexus!
Source: Blue post on the forums