Story related, final achievement.
After beating the most deceitful and final of the bosses the Mysterious Tinker will give you a few more prompts, do what he says to repair the teleporter where you first arrived in the Underworld and activate it to return home.
I’m being vague for spoiler reasons and because it’s just brain dead simple. Press
to access your quest log if you need extra hints and you can always find the Mysterious Tinker on your map.