  The emergence of scalar Higgs-type amplitude modes in systems where symmetry
is spontaneously broken has been a highly successful, paradigmatic description
of phase transitions, with implications ranging from high-energy particle
physics to low-energy condensed matter systems. Here, we uncover two successive
high temperature phase transitions in the pyrochlore magnet Nd$_2$Ru$_2$O$_7$
at $T_{\mathrm{N}} = 147$ K and $T^* = 97$ K, that lead to giant phonon
instabilities and culminate in the emergence of a highly coherent excitation.
This coherent excitation, distinct from other phonons and from conventional
magnetic modes, stabilizes at a low energy of 3 meV. We assign it to a
collective Higgs-type amplitude mode, that involves bond energy modulations of
the Ru$_4$ tetrahedra. Its striking two-fold symmetry, incompatible with the
underlying crystal structure, highlights the possibility of multiple entangled
broken symmetries.
