Portal Artifact

Unreleased Portal Artifact was an unreleased item that only appears in early released BMC. It's not impossible to find it, and the only hint of it is the quest "Ancient Portal Knowledge", which says that it's lost and player needs to reassemble it.

Trivia

 * According to a developer blog on March 29th, 2019, assembling all 5 pieces were meant to lead into an "ultimate city", but it proved to bring some issues with the balance of the game, though Ninja Kiwi has hinted that they may reuse the idea someday. It's unknown if it's still in game's files, as it was removed soon after official release.
 * It has been mentioned in comments from a few NK blogs.