  We propose the construction of LEGEND-1000, the ton-scale Large Enriched
Germanium Experiment for Neutrinoless $\beta \beta$ Decay. This international
experiment is designed to answer one of the highest priority questions in
fundamental physics. It consists of 1000 kg of Ge detectors enriched to more
than 90% in the $^{76}$Ge isotope operated in a liquid argon active shield at a
deep underground laboratory. By combining the lowest background levels with the
best energy resolution in the field, LEGEND-1000 will perform a
quasi-background-free search and can make an unambiguous discovery of
neutrinoless double-beta decay with just a handful of counts at the decay $Q$
value. The experiment is designed to probe this decay with a 99.7%-CL discovery
sensitivity in the $^{76}$Ge half-life of $1.3\times10^{28}$ years,
corresponding to an effective Majorana mass upper limit in the range of 9-21
meV, to cover the inverted-ordering neutrino mass scale with 10 yr of live
time.
