I'm surprised Kralik hasn't mentioned it "officially", but he let the mods and I know the other day he wanted to open it up to the general populace.
Basically, the Dead Pile is where naughty, spammy, and/or dead threads crawl off to when they disappear from the other boards. It's been around since only a week or two into TLHH's existance as a brain child of mine, as I thought it would be good to keep trimmed and/or deleted threads for reference rather than just deleting them outright.
It's also recently become a testing ground for Kralik for new features and such.
Oh, and a stomping ground for Wights. There's that too.