  Heavy Neutral Leptons (HNLs) with masses $\mathcal{O}(0.1 -
1\,\,\text{GeV}/c^{2})$ are promising candidates for the simultaneous
explanation of the smallness of the observed neutrino masses as well as the
matter-antimatter asymmetry in the observable Universe. These particles can be
produced in the decay of hadrons typically produced in a neutrino beamline used
for oscillation experiments, and have sufficient lifetime to propagate to a
near detector, where they decay to observable particles. For the approximation
of a single new mass eigenstate mixing with the Standard Model via the lepton
mixing matrix, a simulation framework based on the GENIE event generator has
been developed. This module is designed to facilitate searches for HNL through
a unified, minimal interface employing a detailed treatment of the kinematics
and dynamics of massive unstable neutrinos, with a transparently organised
suite of physics effects tracking the HNL from its production to its decay.
These mechanisms are expounded on in the current work, underlining the rich
landscape for novel, non-trivial physics that has already been identified in
previous literature. This framework is an ongoing effort to provide a
consistent and comprehensive description of heavy neutrinos from particle
decays. We highlight use cases and future applications of interest to the
accelerator neutrino community.
