It was named after a faint constellation in the southern sky: Pyxis, the Mariner’s Compass. But it helped to steer the mighty Merrill Lynch toward disaster. Barely visible to any but a few inside Merrill, Pyxis was created at the height of the mortgage mania as a sink for subprime securities. Intended for one purpose and operated off the books, this entity and others like it at Merrill helped the bank obscure the outsize risks it was taking.