Shuffle the encounter discard pile into the encounter deck. If there is a set-aside Brood of Yog-Sothoth, the lead investigator reads the following (out loud):

Bleak storm clouds churn overhead and a blanket of rain pelts the countryside. As the downpour grows in intensity, you take refuge in a half-ruined shack nearby. There is a flash of lightning, and in the brief illumination, you spot the outline of something large in the rain. Without warning, the distant trees bend, though nothing seems to be bending them. Moments later, a force with the strength of a truck crashes into your refuge.

Spawn 1 of the set-aside Brood of Yog-Sothoth enemies at the lead investigator's location, if able. Then, each investigator at that location tests (4). The newly spawned Brood of Yog-Sothoth makes an attack against each investigator who fails this skill test.