ATLAS Level-0 Endcap Muon Trigger for HL-LHC

High-Luminosity LHC (HL-LHC) is an upgrade of LHC to achieve higher luminosities, enabling experiments to reach better physics sensitivity. Operation of HL-LHC is scheduled to start in 2026 with an instantaneous luminosity of $7.5 \times 10^{34} \mathrm{cm}^{-2}\mathrm{s}^{-1}$. In order to cope with the luminosity, the trigger and readout systems are planned to be upgraded. The new Level-0 endcap muon trigger system is required to reconstruct muon candidates with an improved momentum resolution to suppress trigger rate while keeping efficiency. That can be achieved by combining signals from thin gap chambers and various subdetectors. Design for the Level-0 endcap muon trigger of the ATLAS experiment at HL-LHC and status of the development are presented.