Diablo III and Diablo III: Reaper of Souls players who log in to Heroes during the three weeks following the release of the upcoming patch will permanently receive Diablo in Heroes of the Storm.
Malthael's Phantom
Starting with Season 4, Diablo III players who reach level 70 with a Seasonal character will receive Malthael’s Phantom in Heroes of the Storm.
Portrait Border & Pennant
Diablo III players who reach Player Level 12 in Heroes of the Storm will receive a new Heroes-themed Portrait Border and Pennant inDiablo III.
No official date on the when the patch will hit Hots, but the Monk will come out on 19 August, so that's a good guess ;p
No need to wait for Season 4. All you have to wait for is the HotS patch, happening tomorrow, but doesn't matter if you're 12 already.
Season 11 Necromancer vids: https://www.youtube.com/c/RavennWo
If you want to receive Diablo in HotS you got till 8 September to log HotS. Rest of the rewards don't seem to have a time limit.
Season 11 Necromancer vids: https://www.youtube.com/c/RavennWo
It's not that hard to get the HS mount, 100 wins vs ai (druid ai) goes very fast, faster then leveling a 70
Season 11 Necromancer vids: https://www.youtube.com/c/RavennWo
For those who play HotS or for those who just want to get the D3 hots Pennant and portrait border:
Diablo III Rewards
No official date on the when the patch will hit Hots, but the Monk will come out on 19 August, so that's a good guess ;p
Season 11 Necromancer vids: https://www.youtube.com/c/RavennWo