  Markarian 817 is a bright and variable Seyfert-1.2 active galactic nucleus
(AGN). X-ray monitoring of Mrk 817 with the Neil Gehrels Swift Observatory in
2022 revealed that the source flux had declined to a lower level than recorded
at any prior point in the then-19-year mission. We present an analysis of deep
XMM-Newton and NuSTAR observations obtained in this low flux state. The spectra
reveal a complex X-ray wind consisting of neutral and ionized absorption zones.
Three separate velocity components are detected as part of a structured
ultra-fast outflow (UFO), with v/c = 0.043 (+0.007,-0.003), v/c = 0.079
(+0.003,-0.0008), and v/c = 0.074 (+0.004,-0.005). These projected velocities
suggest that the wind likely arises at radii that are much smaller than the
optical broad line region (BLR). In order for each component of the outflow to
contribute significant feedback, the volume filling factors must be greater
than f ~ 0.009, f ~ 0.003, and f ~ 0.3, respectively. For plausible,
data-driven volume filling factors, these limits are passed, and the total
outflow likely delivers the fierce feedback required to reshape its host
environment, despite a modest radiative Eddington fraction of lambda ~
0.008-0.016 (this range reflects plausible masses). UFOs are often detected at
or above the Eddington limit; this result signals that black hole accretion has
the potential to shape host galaxies even at modest Eddington fractions, and
over a larger fraction of a typical AGN lifetime. We discuss our findings in
terms of models for disk winds and black hole feedback in this and other AGN.
