It builds more off of Greater Harm, also in HoH. Greater Harm has a range of Close instead of touch, and deals 1d12 per caster lvl, up to 20th. Kinda weird to me, since Harm deals 10 per, not d10. 7th lvl spell.
Mass Harm is a 9th lvl version of Greater Harm, and is a 20ft burst on caster that hits all targets except caster.