  Ta$_2$NiSe$_5$ became one of the most investigated candidate materials for
hosting an excitonic insulator ground state. Many studies describe the
corresponding phase transition as a condensation of excitons breaking a
continuous symmetry. This view got challenged recently pointing out the
importance of the loss of two mirror symmetries at a structural phase
transition that occurs together with the semiconductor-excitonic insulator
transition. For such a scenario an unstable optical zone-center phonon at low
energy is proposed to drive the transition. Here we report on the experimental
observation of such a soft mode behavior using Raman spectroscopy. In addition
we find a novel spectral feature, likely of electronic or joint electronic and
phononic origin, that is clearly distinct from the lattice dynamics and that
becomes dominant at Tc. This suggests a picture of joint structural and
electronic order driving the phase transition.
