Never in horror history has such deliciously atmospheric dread been squandered on such poorly developed characters and plotlines. The premise looks great on paper: An asbestos-removal team tackles an abandoned mental hospital, only to fall prey to — what? Its resident demons? Their own? Session aims for creepy ambiguity, but ends up floundering in vaguesville. There’s more drama — no joke — in watching the paint peel off the leprous walls of the institution; the old pile should fire its agent. It really deserved a better movie.