Observation of $\psi (3686)\to p{K}^{-}{K}_S^0{\bar{\mathrm{\Xi }}}^0+c\mathrm{.}c\mathrm{.}$

Using a sample of $(2.712\pm 0.014)\times {10}^9$ $\psi (3686)$ events collected with the BESIII detector, the decay of $\psi (3686)\to p{K}^{-}{K}_S^0{\bar{\mathrm{\Xi }}}^0+c\mathrm{.}c\mathrm{.}$ is observed for the first time with a statistical significance of $11.5\sigma$. The branching fraction of this decay is measured to be $(2.84\pm 0.40\pm 0.25)\times {10}^{-6}$, where the first and second uncertainties are statistical and systematic, respectively. This measurement extends the experimental information on rare multi-strange $\psi (3686)$ decays and provides an experimental reference for future studies of related decay modes.