Long story, but that quest was actually only meant to be printed in the hardback version of the book (which never happened) - but through a mistake with my publisher, that quest got added to the standard paperback version.
Basically, it is a red level dungeon delve that you can take on during Act 2, although you would probably want to save it until you've done the red quest and before you do the boss battle.
There would be no harm in still playing it, even if you have finished the book. Think of it as a flash back if you want. It might contain some gear upgrades. But it is entirely optional. It was basically unlocked during the Kickstarter as an added bonus.