All around the city marbles are falling from the sky. These marbles are no different from the normal toys children play with other than their origin is unknown, and they do not shatter upon hitting the ground.
Enough people have been wounded (though not severely) by the falling items that the local governance has hired the PCs to investigate the odd source of the marbles and put a stop to them.
PS: Sorry for missing the artifact on Tuesday. Work kicked me in the head hard enough that I just went home and almost collapsed in exhaustion.