  The first-order transition line in the \textit{H-T} phase diagram of
itinerant electron metamagnets terminates at the critical end point-analogous
to the critical point on the gas-liquid condensation line in the \textit{p-T}
phase diagram. To unravel the impact of critical magnetic fluctuations on the
crystal lattice of a metamagnet at the critical end point, we performed an
ultrasonic study of the itinerant electron metamagnet UTe$_2$ across varying
temperatures and magnetic fields. At temperatures exceeding 9 K, a distinct
V-shaped anomaly emerges, precisely centered at the critical field of the
metamagnetic transition in the isothermal field dependence of elastic
constants. This anomaly arises from lattice instability, triggered by critical
magnetic fluctuations via strong magnetoelastic interactions. Remarkably, this
effect is maximized precisely at the critical-end-point temperature.
Comparative measurements of another itinerant metamagnet, UCoAl, reveal
intriguing commonalities. Despite significant differences in the paramagnetic
ground state, lattice symmetry, and the expected metamagnetic transition
process between UTe$_2$ and UCoAl, both exhibit similar anomalies in elastic
properties near the critical end point. These shared aspects may hold
universality for other itinerant electron metamagnets.
