  Right-handed neutrinos (RHNs) provide a natural portal to a dark sector
accommodating dark matter (DM). In this work, we consider that the dark sector
is connected to the standard model only via RHNs and ask how DM can be produced
from RHNs. Our framework concentrates on a rather simple and generic
interaction that couples RHNs to a pair of dark particles. Depending on whether
RHNs are light or heavy in comparison to the dark sector and also on whether
one or both of them are in the freeze-in/out regime, there are many distinct
scenarios resulting in rather different results. We conduct a comprehensive and
systematic study of all possible scenarios in this paper. For illustration, we
apply our generic results to the type-I seesaw model with the dark sector
extension, addressing whether and when DM in this model can be in the freeze-in
or freeze-out regime. Some observational consequences in this framework are
also discussed.
