The Slime Rapture refers to a cluster of Stem Slime hives deep in the Fungal Wastes whose turf's overlap in an area called the Apex Hive.
Imine Green has traveled between these large hives and spun tales for them of Stem City and its horrid inhabitants. The mindless beasts have been told stories of the city torturing Stem Slimes, shredding them of their honor, and treating them as nothing more than dirty animals to be played with. She's been teaching the queens how to speak properly, and the broodmothers are slowly spreading it to their own kin.
Imine has given all these Stem Slimes a prophecy: When the time comes, she will call upon them to unify their hives as one. Under Imine's lead, they will descend upon the Stem Ram inhabitants that have abused slimekind for centuries. They will feast on the ram's meat and never be hungry again.