I don’t think anyone answered this, so…
Once in Eridium Blight, you go up a path in the NE corner of the map. Once you meet Geary, you let him punch you once. He will then jump off the cliff, and the LotR easter egg will spawn. If he keeps going after you after the first punch, the whole “mission” is failed. If you continue up the path, you will find a red chest, and get the “Vault hunter vs Volcano” challenge.
You used to be able to drive all the way from Three Horns to Eridium Blight, but this was patched at some point. These days it’s more of a Banshee run, accompanied by Yakety sax.

