We don't add custom spells. It has to be spells that exists in published d20 sources that can be properly implemented with the game engine.
One of them isn't a custom spell. Mass Raise Dead's origin is in a published BioWare game, ironically. It could only fit in for level 16-20 clerics and diamond requirements are still a thing in PotM. Probably ends up being a gold sink.
The only mass raise dead ability that immediately comes to mind is the capstone for the Monk of the Healing Hand archetype published by Paizo.
True Sacrifice (Su)
At 20th level, in a final selfless act, a monk of the healing hand can draw in his entire ki, which then explodes outward in a 50-foot-radius emanation. All dead allies within the emanation are brought back to life, as if they were the subject of a true resurrection spell with a caster level equal to the monk’s level. When the monk does this, he is truly and utterly destroyed. A monk destroyed in this way can never come back to life, not even by way of a wish or miracle spell or by the power of a deity. Furthermore, the monk’s name can never be spoken or written down again. All written mentions of his name become nothing more than a blank space.
This ability replaces perfect self.
And while Baldur's Gate is a Bioware game that is based on a DnD system, I can't recall any official WotC published works containing a mass raise dead/mass resurrection spell.