Rosie died ~ 1910s based on context clues.

Was in hell... 2 decades?

Was powerful enough for Alastor to summon... And powerful enough to make him 'the most powerful sinner' in return for an unstated task. (And maybe to coordinate his timely demise to start his mission)

And Alastor believes that he can somehow complete that task by keeping Charlie safe and... Giving himself to the Vees... Something something war with heaven that is supposedly happening bc of exterminations that started when Lilith and Alastor disappeared....

Who the fuck is Rosie?

And what did she want with Alastor a century ago, that is applicable to this ongoing war?