The Hearthstone of the Flame, a new cosmetic Hearthstone with a fairly explosive visual, will be available as a potential reward from Amirdrassil, the Dream's Hope raid. The Hearthstone of the Flame has a chance to drop from Larodar, Keeper of the Flame at all difficulties as bonus loot.
Amirdrassil Raid Collectable Rewards Guide — Mounts, Titles, Cosmetics
What truly makes Hearthstone of the Flame unique is the spectacular pyrotechnic visual that accompanies the casting of the Hearthstone. Several distant columns of flame explode slowly converging on the player eventually engulfing their position as the caster teleports away.
Larodar, Keeper of the Flame is a mid-raid encounter meaning players should have many attempts at acquiring this new cosmetic hearthstone, especially since it does appear to drop on all raid difficulties.
If you're a pyromaniac or just want to round out your best Druid of the Flame transmog, then be sure to farm out this new hearthstone.