  We present \chandra high energy transmission grating spectra of the
narrow-line Seyfert-1 galaxy Ark 564. The spectrum shows numerous absorption
lines which are well modeled with low velocity outflow components usually
observed in Seyfert galaxies \citep{Gupta2013}. There are, however, some
residual absorption lines which are not accounted for by low-velocity outflows.
Here we present identifications of the strongest lines as $K\alpha$ transitions
of \oviin (two lines) and \ovin at outflow velocities of $\sim 0.1c$. These
lines are detected at $6.9\sigma$, $6.2\sigma$, and $4.7\sigma$ respectively
and cannot be due to chance statistical fluctuations. Photoionization models
with ultra-high velocity components improves the spectral fit significantly,
providing further support for the presence of relativistic outflow in this
source. Without knowing the location of the absorber, its mass and energy
outflow rates cannot be well constrained; we find $\dot{E}(outflow)/L_{bol}$
lower limit of $\geq 0.006$% assuming a bi-conical wind geometry. This is the
first time that absorption lines with ultra-high velocities are unambiguously
detected in the soft X-ray band. The presence of outflows with relativistic
velocities in AGNs with Seyfert-type luminosities is hard to understand and
provides valuable constraints to models of AGN outflows. Radiation pressure is
unlikely to be the driving mechanism for such outflows and magneto-hydrodynamic
may be involved.
