  We spectroscopically confirm the $M_{\rm UV} = -20.5$ mag galaxy
GHZ2/GLASS-z12 to be at redshift $z=12.34$. The source was selected via NIRCam
photometry in GLASS-JWST ERS data, providing the first evidence of a surprising
abundance of bright galaxies at $z \gtrsim 10$. The NIRSpec PRISM spectrum
shows detections of N IV, C IV, He II, O III, C III, O II, and Ne III lines,
and the first detection at high-redshift of the O III Bowen fluorescence line
at 3133 {\AA} rest-frame. The prominent C IV line with rest-frame equivalent
width (EW) $\approx 46$ {\AA} puts GHZ2 in the category of extreme C IV
emitters. GHZ2 displays UV lines with EWs that are only found in active
galactic nuclei (AGNs) or composite objects at low/intermediate redshifts. The
UV line-intensity ratios are compatible both with AGNs and star formation in a
low-metallicity environment, with the low limit on the [Ne IV]/[N IV] ratio
favoring a stellar origin of the ionizing photons. We discuss a possible
scenario in which the high ionizing output is due to low metallicity stars
forming in a dense environment. We estimate a metallicity $\lesssim 0.1 Z/{\rm
Z}_{\odot}$, a high ionization parameter logU $> -2$, a N/O abundance 4--5
times the solar value, and a subsolar C/O ratio similar to the recently
discovered class of nitrogen-enhanced objects. Considering its abundance
patterns and the high stellar mass density ($10^4$~M$_{\odot}$~pc$^{-2}$), GHZ2
is an ideal formation site for the progenitors of today's globular clusters.
The remarkable brightness of GHZ2 makes it a ``Rosetta stone'' for
understanding the physics of galaxy formation within just 360 Myr after the Big
Bang.
