This is how you obtain the new Artifact of Tempus.
Risk of Rain Returns How to Unlock the Artifact of Tempus
To unlock the Artifact of Tempus, all you need to do is reach the Temple of the Elders (Stage 5) and grab the artifact in under 16 minutes. Be aware, you need to actually collect the artifact before 16:00, otherwise it appears to despawn.
So far, every time I saw the artifact it was in one of the large vertical tower section, in front of what appears to be a hole in the wall.
That’s it. I’ve seen it as a couple of different characters, on both Rainstorm and Monsoon difficulties, so I do not believe there are any other requirements for it to spawn.