Blank rune thing?

[Burzum]

Found a large blank rune down a ton of floor break tunnels.
Whats it for?

[4thQueen]

Brb using imagination cause no pics

[Burzum]

http://steamcommunity.com/sharedfiles/filedetails/?id=891383852

[4thQueen]

Oh, that one. You need to equip the Spore Shroom charm to read that and other mushroom related signs. That one serves as a hintpost for the Mr Mushroom ending.

tl;dr don't worry about it, it's part of the joke ending

[Burzum]

Cheers dude. I've gotten very lost in this miserable place, that I assume is late game. Time to go back.

[bad]

Yes, very much late game. Once you've explored most things, you could go for this. It's a small secret which unlocks a post-credit scene and an achievement. Pretty sure you also have to kill the final boss once to start doing this secret.

[HighLanderPony]

Dangit, another thing I thought I knew about so I checked it, and now it's spoiled. :P