  We present a study of two-photon pathways for the transfer of NaCs molecules
to their rovibrational ground state. Starting from NaCs Feshbach molecules, we
perform bound-bound excited state spectroscopy in the wavelength range from
900~nm to 940~nm, covering more than 30 vibrational states of the $c \,
^3\Sigma^+$, $b \, ^3\Pi$, and $B \, ^1\Pi$ electronic states. Analyzing the
rotational substructure, we identify the highly mixed $c \, ^3\Sigma^+_1 \,
|v=22 \rangle \sim b \, ^3\Pi_1 \, | v=54\rangle$ state as an efficient bridge
for stimulated Raman adiabatic passage (STIRAP). We demonstrate transfer into
the NaCs ground state with an efficiency of up to 88(4)\%. Highly efficient
transfer is critical for the realization of many-body quantum phases of
strongly dipolar NaCs molecules and high fidelity detection of single
molecules, for example, in spin physics experiments in optical lattices and
quantum information experiments in optical tweezer arrays.
