Ameratsu's Furnace
What is it? Where can I find more info? I was mainly looking for places in the Shadowlands to use in a campaign.

There's some information on in it in Fortunes and Winds (pg. 38). I know there are other sources, but I can't remember where they are off the top of my head. I know there was the Anvil of Despair Clan War Module Pack, which has a bit more information but I don't have that with me. I also seem to recall there was some mentioning of Amaterasu's Furnace in the Imperial and Jade fictions.

It’s a volcanic lave field in the Shadowlands that is a portal to some of the darker spirit realms. Matsu Hiroru went into it in an attempt to rescue Doji Shizue’s soul from Meido. Hiroru became a Gaki.

Matsu Hiroru went into it in an attempt to rescue Doji Shizue’s soul from Meido. Hiroru became a Gaki. Yep, I do recall that now. Checking the entry of page 38, it basically has everything that you mentioned in addition to the story of its creation due to the death of Hida and Hida Atarasi.

I thought it was an area purified of the Shadowlands taint? Or is it untainted, but still a portal to dark realms?

Okay. I was imagining it as a flower filled meadow on a dormant Volcano in the Shadowlands. I guess that changes the adventure around a bit... :)

No, it is a pretty nasty place and is home to a spirit portal to Gaki-do.