Rogue (Croak #3) by Gina Damico.mobi

Lex is a teenage Grim Reaper with the power to Damn souls, and it’s getting out of control. She’s a fugitive, on the run from the maniacal new mayor of Croak and the townspeople who want to see her pay the price for her misdeeds. Uncle Mort rounds up the Junior Grims to flee Croak once again, but this time they’re joined by Grotton, the most powerful Grim of all time. Their new mission is clear: Fix his mistakes, or the Afterlife will cease to exist, along with all the souls in it.　

The gang heads for Necropolis, the labyrinth-like capital city of the Grimsphere. There, they discover that the Grimsphere needs a reboot. To do that, the portals to the Afterlife must be destroyed…but even that may not be enough to fix the damage. Things go from bad to worse, and when at last the fate of the Afterlife and all the souls of the Damned hang in the balance, it falls to Lex and her friends to make one final, impossible choice.