For intro posts, right now, I'm thinking we just keep it simple and introduce our characters, their role within the tribes, etc. Likewise, maybe we can start showing how the new wave of mutants are affecting them / how these characters are responding to the warning signs.
Bear in mind, the source of the mutants is on a stoney island all the way in the south (it's marked with a black skull on the map somewhere), so tribes that are farther away may have more subtle signs (e.g, they hear from rumors about travelers being attacked by mutants, or just find a couple of stragglers on the outskirts of their territory, etc). Tribes that are closer to the isle, on the contrary, might have more obvious signs (they see more of the black buds referenced in the intro post, dead nature, more mutants, etc.)
Also, let me know if you'd like a higher-res version of the map, I can send you a link!