  In November 2020, a new, bright object, eRASSt J234402.9$-$352640, was
discovered in the second all-sky survey of SRG/eROSITA. The object brightened
by a factor of at least 150 in 0.2--2.0 keV flux compared to an upper limit
found six months previous, reaching an observed peak of $1.76_{-0.24}^{+0.03}
\times 10^{-11}$ erg cm$^{-2}$ s$^{-1}$. The X-ray ignition is associated with
a galaxy at $z=0.10$, making the peak luminosity log$_{10}(L_{\rm
0.2-2keV}/[\textrm{erg s}^{-1}])$=$44.7\pm0.1$. Around the time of the rise in
X-ray flux, the nucleus of the galaxy brightened by approximately 3 mag. in
optical photometry, after correcting for the host. We present data from Swift,
XMM-Newton, and NICER, which reveal a very soft spectrum as well as strong
0.2--2.0 keV flux variability on multiple timescales. Optical spectra taken in
the weeks after the ignition event show a blue continuum with broad, asymmetric
Balmer emission lines, and high-ionisation ([OIII]$\lambda\lambda$4959,5007)
and low-ionisation ([NII]$\lambda$6585, [SII]$\lambda\lambda$6716,6731) narrow
emission lines. Following the peak in the optical light curve, the X-ray, UV,
and optical photometry all show a rapid decline. The X-ray light curve shows a
decrease in luminosity of $\sim$0.45 over 33 days and the UV shows a drop of
$\sim$0.35. eRASSt J234402.9$-$352640 also shows a brightening in the
mid-infrared, likely powered by a dust echo of the luminous ignition. We find
no evidence in Fermi-LAT $\gamma$-ray data for jet-like emission. The event
displays characteristics of a tidal disruption event (TDE) as well as of an
active galactic nucleus (AGN), complicating its classification. Based on the
softness of the X-ray spectrum, the presence of high-ionisation optical
emission lines, and the likely infrared echo, we find that a TDE within a
turned-off AGN best matches our observations.
