The most likely way to introduce heros into the locked off realm of Underlight is to have Summer Breeze misdirect them on a spherewalk. Having a previous quest end with an uncontrolled release of powerful magic or stepping through an unstable gate could also provide probable reasons, perhaps helped along by the summons. In general, only spherewalkers who are in transit between spheres will be redirected, either an established group going together, or (in the case of a new group) a seemingly random collection of spherewalkers who happened to be going through a gate at the time, who will be quite surprised to appear together.
There are essentially two places that the heros could appear. Either in Summer Breeze's presence at the temple of Shine, or at the old gate in The City. If they appear at the temple, show them the vision card of Summer Breeze; her expression in the picture is just perfect for this situation. As soon as they arrive, she will demand something to the effect 'Are you the heros I summoned?' In the ensuing conversation, she will first attempt to see if they have the secret of war, and then expect them do something about the city-dwellers rule.
Starting them out in the city could be an interesting exercise indeed. Although I try to avoid 'good guys vs. bad guys' as much as possible, the followers of Shimmer come out closer to bad guys. Thus giving the heros their side of the story first could result in an interesting 'double take' when they realize what is going on. The principle problem with starting them in The City is of course getting them out. This would probably work best with either several days before the ritual, or none. Several days so they have time to explore, find the tomb and other clues, get curious, and get to the temple of Shine and back. That night so that it happens and they then have scramble around for information and find Summer Breeze to set things right.