  We advocate for the construction of a new detector element at the LHCb
experiment, designed to search for displaced decays of beyond standard model
long-lived particles, taking advantage of a large shielded space in the LHCb
cavern that is expected to soon become available. We discuss the general
features and putative capabilities of such an experiment, as well as its
various advantages and complementarities with respect to the existing LHC
experiments and proposals such as SHiP and MATHUSLA. For two well-motivated
beyond Standard Model benchmark scenarios -- Higgs decay to dark photons and
$B$ meson decays via a Higgs mixing portal -- the reach either complements or
exceeds that predicted for other LHC experiments.
