Hiruko the Goblin

A school has been built on one of the Gates of Hell, behind which hordes of demons await the moment they will be free. Hiruko is a goblin sent to Earth on a reconnaissance mission. He decapitates students in order to assemble their heads on the demons' spider-like bodies. Eventually, it falls to an archaeology professor at the school to investigate the gory deaths, and discover some way to stop this demonic onslaught. (official distributor synopsis)