  Magnetars are slowly rotating neutron stars that possess the strongest
magnetic fields ($10^{14}-10^{15} \mathrm{G}$) known in the cosmos. They
display a range of transient high-energy electromagnetic activity. The
brightest and most energetic of these events are the gamma-ray bursts (GRBs)
known as magnetar giant flares (MGFs), with isotropic energy
$E\approx10^{44}-10^{46} \mathrm{erg}$. There are only seven detections
identified as MGFs to date: three unambiguous events occurred in our Galaxy and
the Magellanic Clouds, and the other four MGF candidates are associated with
nearby star-forming galaxies. As all seven identified MGFs are bright at Earth,
additional weaker events remain unidentified in archival data. We conducted a
search of the Fermi Gamma-ray Burst Monitor (GBM) database for candidate
extragalactic MGFs and, when possible, collected localization data from the
Interplanetary Network (IPN) satellites. Our search yielded one convincing
event, GRB 180128A. IPN localizes this burst with NGC 253, commonly known as
the Sculptor Galaxy. This event is the second MGF in modern astronomy to be
associated with this galaxy and the first time two bursts are associated with a
single galaxy outside our own. Here, we detail the archival search criteria
that uncovered this event and its spectral and temporal properties, which are
consistent with expectations for a MGF. We also discuss the theoretical
implications and finer burst structures resolved from various binning methods.
Our analysis provides observational evidence for an eighth identified MGF.
