Currently, Blizzard is having a crossover event for Diablo 3 and Heroes of the Storm.
Immediately following the next Heroes patch, any player with a Diablo III license on their Battle.net account will receive the Diablo Hero in Heroes of the Storm for free.
You will be able to obtain your Diablo Hero until September 8th at 10am PDT. This goes for individuals who already own Diablo 3 or are going to purchase it soon.
If you’re willing or already put in the time to reach Level 12 in Heroes of the Storm, you will be rewarded a unique pennant and portrait frame for use in Diablo 3.
On the other side of the spectrum, any Players that reach Lvl. 70 with a Seasonal Hero in Season 4 will receive Malthael’s Phantom Mount for use in Heroes of the Storm.