Smolderheart drop from Sire Ladinas in Revendreth. To trigger Sire Ladinas you need Pick up a Remnant of Light around The Desiccation and use Light Impalement on Crazed Ash Ghoul. When the rare spawn Crazed Ash Ghoul will speak the following message in your chat: "The Light... buuurns!". Rewards: Smolderheart (not 100% drop rate) & Pocket Embers.
Here you can buy Smolderheart Toy Boost for EU or US Servers.
You'll get guide for completing Smolderheart Toy Boost