Compare it to the 9th level WIZARD spells (and I remind you Wizards are supposed to be the one doing badass damage spells, not Clerics) with comparable function, Wail of the Banshee and Wierd.
Wierd too beats Death Immunity, but is stopped by Clarity and requires two saves, a will and a fort. Normal DC, no +3.
Wail of the Banshee dosen't beat death immunity, and dosen't get +3 like Implosion.
Implosion is definitely more powerful than it should be, IMO.