  We present observations of Swift J1112.2-8238, and identify it as a candidate
relativistic tidal disruption flare (rTDF). The outburst was first detected by
Swift/BAT in June 2011 as an unknown, long-lived (order of days) $\gamma$-ray
transient source. We show that its position is consistent with the nucleus of a
faint galaxy for which we establish a likely redshift of $z=0.89$ based on a
single emission line that we interpret as the blended [OII]$\lambda3727$
doublet. At this redshift, the peak X/$\gamma$-ray luminosity exceeded
$10^{47}$ ergs s$^{-1}$, while a spatially coincident optical transient source
had $i^{\prime} \sim 22$ (M$_g \sim -21.4$ at $z=0.89$) during early
observations, $\sim 20$ days after the Swift trigger. These properties place
Swift J1112.2-8238 in a very similar region of parameter space to the two
previously identified members of this class, Swift J1644+57 and Swift
J2058+0516. As with those events the high-energy emission shows evidence for
variability over the first few days, while late time observations, almost 3
years post-outburst, demonstrate that it has now switched off. Swift
J1112.2-8238 brings the total number of such events observed by Swift to three,
interestingly all detected by Swift over a $\sim$3 month period ($<3\%$ of its
total lifetime as of March 2015). While this suggests the possibility that
further examples may be uncovered by detailed searches of the BAT archives, the
lack of any prime candidates in the years since 2011 means these events are
undoubtedly rare.
