Make it tangible.
If the heroes fail to obtain the artifacts for the powerful entity, then they are will be slain and some other bad thing happens too. Say, their home city will be destroyed.
The wish is just... meh.

Have less "contestants" and force them to work together.

Thing is, the outsider is just playing them to get the artifacts that he himself cannot obtain for reason XY or Z.

But what does he want them for? Doesn't have to be anything nefarious, but it could be.

The Outsider could also be scrying everything that they do and using their attempts to get the artifacts as a reality tv show for his other outsider chums.

Previously on That's My Artifact.......