"Heavenly host" refers to a large group of angels or heavenly beings. It describes the celestial army or assembly of heavenly creatures who serve and worship God. Full definition
When a spiritual photograph turns up, suggesting the Book of Shadows may now reside in the tattered remains of Heavenly Host, Ayumi becomes obsessed with finding it so she can make one last attempt to set things right. (marvelousgames.com)
The final installment of the supernatural story that began with Corpse Party and continued in Corpse Party: Book of Shadows, Corpse Party: Blood Drive ups the ante by allowing players to explore Heavenly Host Elementary in full 3D for the first time, with all of its formerly 2D environments meticulously recreated and greatly expanded. (xseedgames.com)
It's not part of the main Heavenly Host arc, but rather a diversion from it. (xseedgames.com)