Found a hidden area; here is a copy from the GrimToolsMap chat:

[tean101]

Hello @Dammitt,

I found a very, very well-hidden spot, a cave called “Mammoth Cave”, eastern Pilgrims Rest Rift, that isn’t listed on the GrimToolsMap and isn’t marked as accessible.

There are a few things inside, such as a dynamite chest. I haven’t yet figured out how to open the monolith to access the locked boss chest.

[BMag108]

if you’re on about the chest oh Bal-Eigen, you light the braziers in the correct order then it opens up/ the curse goes away.

[marfig]

The correct order is dictated by the lore notes that you collected around Asterkan telling the story of this hero.