  We report on the discovery of a peculiar Broad Absorption Line (BAL) quasar
identified in our Gaia-assisted survey of red quasars. The systemic redshift of
this quasar was difficult to establish due to the absence of conspicuous
emission lines. Based on deep and broad BAL troughs (at least SiIV, CIV, and
AlIII), a redshift of z=2.41 was established under the assumption that the
systemic redshift can be inferred from the red edge of the BAL troughs.
However, we observe a weak and spatially-extended emission line at 4450 AA most
likely due to Lyman-alpha emission, which implies a systemic redshift of z=2.66
if correctly identified. There is also evidence for the onset of Lyman-alpha
forest absorption bluewards of 4450 AA and evidence for H-alpha emission in the
K-band consistent with a systemic redshift of z=2.66. If this redshift is
correct, the quasar is an extreme example of a detached low-ionisation BAL
quasar. The BAL lines must originate from material moving with very large
velocities ranging from 22000 to 40000 km/s. To our knowledge, this is the
first case of a systemic-redshift measurement based on extended Lyman-$\alpha$
emission for a BAL quasar, a method that should also be useful in cases of
sufficiently distant BL Lac quasars without systemic-redshift information.
