I would handle it like this:
- Place some kind of “shadow” marker next to the spellcaster.
- As long as the marker is there, allies within 6” may hide as per the usual rules with the difference that you don’t need any terrain in order to hide as usual because the “shadow” of the spell provides this.
- Allies within 6” of the marker benefit from cover against shooting (this is irrelevant once the models choose to actually hide in the shadow)
- Remove the marker once an enemy moves within 6” of the marker.
- Remove the marker at the beginning if the Shadow Warriors’ player’s turn.