The design of the Level-0 endcap muon trigger for the ATLAS experiment at High Luminosity LHC and the status of the development are presented. High Luminosity LHC is planned to start the operation in 2027 with an instantaneous luminosity of $7.5\times 10^{34}\;\mathrm{cm^{-2}s^{-1}}$. In order to cope with the proton-proton collision rate higher than that of LHC, the trigger and readout systems need to be replaced. The new Level-0 endcap muon trigger system is required to reconstruct muon candidates with an improved momentum resolution to suppress the trigger rate with keeping efficiency. That can be achieved by combining signals from various subdetectors, thin gap chambers, resistive plate chambers, micromesh gaseous detectors, and scintillator-steel hadronic calorimeters, to form more offline-like tracks. The performance of the Level-0 endcap muon trigger was evaluated with software and the firmware development is ongoing.