On Prime a 30-mile wide piece of another mortal world manifests in the Pit. The Navigator schedules some by Treaty with worlds we've been to previously. Sometimes we get a "Blind Prime". The number varies from year to year because of variations in the quantum phlogiston and currents in the Astral Sea. Blind Primes are as unpredictable as Shatter and can be just as dangerous.
Prime's me second favorite day of the week! I've seen it all: jungles full of dinos, cold seas full of glowing fish, a city of glass and metal towers a thousand feet tall! I either get to visit with old friends on other worlds, I get to exercise me old Salvager license for some profit, or I get a chance to fight off whatever idiots think they can invade US!.
On the fourth day the Pit manifests another mortal world. Next to Shatter, Prime is the most unpredictable day. The Orrery is not always exact and the folk of the Pit, the Kirinalos, have discovered that mortal worlds and mortal people can be just as dangerous as the demons of the Abyss.
Kirinalos wait with cautious anticipation for the horns to sound. The Navigator steers the Orrery and another world of the Prime appears. He has a few seconds to choose the manifestation point. Treaty worlds have a designated point, but if it's a strange plane he errs on the side of caution. But somestimes what appears safe is anything but.
Crystal Spheres and Strange Horizons
Most worlds that manifest come from crystal spheres similar to Erathia. They usually have differing combinations of familiar races and monsters. They are worlds with names like Toril and Oerth, Athas and Krynn.
Some worlds differ oddly. Populated by humans or strange creatures with wildly varying levels of technology. A stunning variety of places from across the dimensions manifest with names like Coruscant and London, Rivendell and Gotham.