  We report a Chandra high-energy grating detection of a narrow, redshifted
absorption line superimposed on the red wing of a broad Fe K line in the
$z=0.297$ quasar E1821+643. The absorption line is detected at a confidence
level, estimated by two different methods, in the range ~2-3 sigma. Although
the detection significance is not high enough to exclude a non-astrophysical
origin, accounting for the absorption feature when modeling the X-ray spectrum
implies that the Fe-K emission line is broad, and consistent with an origin in
a relativistic accretion disk. Ignoring the apparent absorption feature leads
to the conclusion that the Fe-K emission line is narrower, and also affects the
inferred peak energy of the line (and hence the inferred ionization state of
Fe). If the absorption line (at ~6.2 keV in the quasar frame) is real, we argue
that it could be due to gravitationally redshifted Fe XXV or Fe XXVI resonance
absorption within ~10-20 gravitational radii of the putative central black
hole. The absorption line is not detected in earlier low-energy grating
observations, but is not unequivocally ruled out by these data. The Chandra
high-energy grating Fe K emission line is consistent with an origin
predominantly in Fe I-XVII or so. In a previous LEG observation the line was
double-peaked, at ~6.4 keV and ~6.9 keV (H-like Fe). Such a wide range in
ionization state of Fe is not ruled out by the HEG data and an earlier ASCA
observation, and is suggestive of a complex structure for the line-emitter.
