Jamieson, Fausset and Brown on Isaiah 14:15

15. to hell--to Sheol (Is 14:6), thou who hast said, "I will ascend into heaven" (Mt 11:23).

sides of the pit--antithetical to the "sides of the north" (Is 14:13). Thus the reference is to the sides of the sepulcher round which the dead were arranged in niches. But Maurer here, as in Is 14:13, translates, "the extreme," or innermost parts of the sepulchre: as in Eze 32:23 (compare 1Sa 24:3).