  We study the hybrid mesons with the exotic quantum number $I^GJ^{PC} =
0^+1^{-+}$ and investigate their decays into the $\eta \eta^\prime$, $a_1(1260)
\pi$, $f_1(1285) \eta$, $f_1(1420) \eta$, $K^*(892) \bar K$, $K_1(1270) \bar
K$, and $K_1(1400) \bar K$ channels. We find that the QCD axial anomaly
enhances the decay width of the $\eta \eta^\prime$ channel although this mode
is strongly suppressed by the small $P$-wave phase space. Our results support
the interpretation of the $\eta_1(1855)$ recently observed by BESIII as the
$\bar s s g$ hybrid meson of $I^GJ^{PC}=0^+1^{-+}$. The QCD axial anomaly
ensures the $\eta \eta^\prime$ decay mode to be a characteristic signal of the
hybrid nature of the $\eta_1(1855)$.
