Reach the 5th stage and find a place with 9 pedestals and a portal under the map.
Change the figures on pedestals like this:
Then activate the laptop in front of these pedestals and go into the activated portal.
There you need to "kill" the boss by activating "keys" that drop from enemies. Roughly 5 keys is enough. Be afraid of doubled fall damage + you can die from falling.
Then you should pick up the artifact and leave through the portal.