  The perovskite antiferromagnetic ($T_{\rm N}$ $\sim$ 220 K) insulator
EuNiO$_3$ undergoes at ambient pressure a metal-to-insulator transition at
$T_{\rm MI}$ = 460 K which is associated with a simultaneous
orthorhombic-to-monoclinic distortion, leading to charge disproportionation. We
have investigated the change of the structural and magnetic properties of
EuNiO$_3$ with pressure (up to $\sim$ 20 GPa) across its quantum critical point
(QCP) using low-temperature synchrotron angle-resolved x-ray diffraction and
$^{151}$Eu nuclear forward scattering of synchrotron radiation, respectively.
With increasing pressure we find that after a small increase of $T_{\rm N}$
($p$ $\leq$ 2 GPa) and the induced magnetic hyperfine field $B_{\rm hf}$ at the
$^{151}$Eu nucleus ($p$ $\leq$ 9.7 GPa), both $T_{\rm N}$ and $B_{\rm hf}$ are
strongly reduced and finally disappear at $p_{\rm c}$ $\cong$ 10.5 GPa,
indicating a magnetic QCP at $p_{\rm c}$. The analysis of the structural
parameters up to 10.5 GPa reveals no change of the lattice symmetry within the
experimental resolution. Since the pressure-induced insulator-to-metal
transition occurs at $p_{\rm IM}$ $\cong$ 6 GPa, this result implies the
existence of an antiferromagnetic metallic state between 6 and 10.5 GPa. We
further show from the analysis of the reported high pressure electrical
resistance data on EuNiO$_3$ at low-temperatures that in the vicinity of the
QCP the system behaves as non-Fermi-liquid, with the resistance changing as
$T^{\rm n}$, with n=1.6, whereas it becomes a normal Fermi-liquid, n = 2, for
pressures above $\sim$15 GPa. On the basis of the obtained data a magnetic
phase diagram in the ($p$, $T$) space is suggested.
