  We present a conceptual study of a large format imaging spectrograph for the
Large Submillimeter Telescope (LST) and the Atacama Large Aperture
Submillimeter Telescope (AtLAST). Recent observations of high-redshift galaxies
indicate the onset of earliest star formation just a few 100 million years
after the Big Bang (i.e., z = 12--15), and LST/AtLAST will provide a unique
pathway to uncover spectroscopically-identified first forming galaxies in the
pre-reionization era, once it will be equipped with a large format imaging
spectrograph. We propose a 3-band (200, 255, and 350 GHz), medium resolution (R
= 2,000) imaging spectrograph with 1.5 M detectors in total based on the KATANA
concept (Karatsu et al. 2019), which exploits technologies of the integrated
superconducting spectrometer (ISS) and a large-format imaging array. A 1-deg2
drilling survey (3,500 hr) will capture a large number of [O III] 88 um (and [C
II] 158 um) emitters at z = 8--9, and constrain [O III] luminosity functions at
z > 12.
