  As the earliest relics of star formation episodes of the Universe, the most
massive galaxies are the key to our understanding of the stellar population,
cosmic structure, and SMBH evolution. However, the details of their formation
histories remain uncertain. We address these problems by planning a large
survey sample of 101 ultramassive galaxies ($z\le0.3$,
$|\delta+24^{\circ}|<45$\deg, $|b|>8$\deg), including 76\% ellipticals, 17\%
lenticulars, and 7\% spirals brighter than $M_K\le-27$~mag (stellar mass
$2\times10^{12}\lesssim M_\star\lesssim5\times10^{12}$~\Msun) with ELT/HARMONI.
Our sample comprises diverse galaxy environments ranging from isolated to
dense-cluster galaxies. The primary goals of the project are to (1) explore the
stellar dynamics inside galaxy nuclei and weigh SMBHs, (2) constrain the black
hole scaling relations at the highest mass, and (3) probe the late-time
assembly of these most massive galaxies through the stellar population and
kinematical gradients. We describe the survey, discuss the distinct
demographics and environmental properties of the sample, and simulate their
HARMONI $I_z$, $I_z+J$, and $H+K$-band observations via combining the inferred
stellar-mass models from Pan-STARRS observations, an assumed synthetic spectrum
of stars, and SMBHs with masses estimated based on different black hole scaling
relations. Our simulations produce excellent state-of-art IFS and stellar
kinematics ($\Delta V_{\rm rms}\lesssim1.5$\%) in a relatively short exposure
time. We use these stellar kinematics in combination with the JAM to
reconstruct the SMBH mass and its error using the MCMC simulation. Thus, these
simulations and modelings can be benchmarks to evaluate the instrument models
and pipelines dedicated to HARMONI to exploit the unprecedented capabilities of
ELT.
