Even though this seems like a great deal, you are going to have to decide quickly if you want to take it. The announcement also mentioned the expiration date of the offer and that is going to be the morning of September 8th. That gives you just more than two weeks to take advantage of it.
In order to take advantage of the deal, all you have to do is log into the Heroes of the Storm game and accept your free Diablo character. If you were able to get to level 12 in Heroes of the Storm, you can also unlock a pennant and a portrait frame for your account.
There is plenty of speculation about the cross promotion with some leaning towards the possible expansion for Diablo 3 that is reportedly coming very soon. The expansion was rumored to be coming around November when Blizzard holds its annual convention called BlizzCon. Until then, there has not been any confirmation about the expansion.