Spirits Within, Atonement, and Chi Blast are all in the same quasi-category as breath damage; they're not breath damage themselves, though spirits within is, basically, since almost universally breath damage is based on HP, but just about everything that resists breath damage specifically resists SW/Atonement/Chi Blast as well. I don't know/remember/care if anyone ever figured out whether the two damage types are distinct from one another, but basically, breath damage ignores MDB, and is a unique damage category. Honestly, the easiest way to figure out wtf requiescat does is to go find one of the thousand NMs that make SW/atonement/chiblast utterly useless (IE: I've yet to do a 11~12 boost chi blast against an abyssean NM and break 100 damage), and see if requiescat does even less damage than usual. If it doesn't, than it's a new, unique category unto itself.