Heart On Fire

We have the shards, but now they must be combined. There's a forge here, but even its fires aren't hot enough for our purposes. However, there is a demon nearby that burns hot as the sun. Gul'dan, or perhaps Razuun, has managed to summon an infernal. The thing calls itself Krosnis, which means "furnace." The demon's heart burns hotly enough. All you need do is collect it.