  We present $\texttt{ExTraSS}$ (EXplosive TRAnsient Spectral Simulator), a
newly developed code aimed at generating 3D spectra for supernovae in the
nebular phase by using modern multi-dimensional explosion models as input. It
is well established that supernovae are asymmetric by nature, and that the
morphology is encoded in the line profiles during the nebular phase, months
after the explosion. In this work, we use $\texttt{ExTraSS}$ to study one such
simulation of a $3.3\,M_\odot$ He-core explosion
($M_\text{ejecta}=1.3\,M_\odot$, $E_\text{kin}=1.05\times10^{51}\,$erg)
modelled with the $\texttt{Prometheus-HotB}$ code and evolved to the homologous
phase. Our code calculates the energy deposition from the radioactive decay of
$^{56}$Ni $\rightarrow$ $^{56}$Co $\rightarrow$ $^{56}$Fe and uses this to
determine the Non-Local-Thermodynamic-Equilibrium temperature, excitation and
ionization structure across the nebula. From the physical condition solutions
we generate the emissivities to construct spectra depending on viewing angles.
Our results show large variations in the line profiles with viewing angles, as
diagnosed by the first three moments of the line profiles; shifts, widths, and
skewness. We compare line profiles from different elements, and study the
morphology of line-of-sight slices that determine the flux at each part of a
line profile. We find that excitation conditions can sometimes make the
momentum vector of the ejecta emitting in the excited states significantly
different from that of the bulk of the ejecta of the respective element, thus
giving blueshifted lines for bulk receding material, and vice versa. We compare
the 3.3 $M_\odot$ He-core model to observations of the Type Ib supernova SN
2007Y.
