  Radio relics are diffuse synchrotron sources in the outskirts of merging
galaxy clusters energized by the merger shocks. In this paper, we present an
overview of the radio relics in massive cluster mergers identified in the new
TNG-Cluster simulation. This is a suite of magnetohydrodynamical cosmological
zoom-in simulations of 352 massive galaxy clusters with $M_{\rm 500c}=
10^{14.0-15.3}\rm~M_{\odot}$ sampled from a 1 Gpc-size cosmological box. The
simulations are performed using the moving-mesh code AREPO with the galaxy
formation model and high numerical resolution consistent with the TNG300 run of
the IllustrisTNG series. We post-process the shock properties obtained from the
on-the-fly shock finder to estimate the diffuse radio emission generated by
cosmological shockwaves for a total of $\sim300$ radio relics at redshift
$z=0-1$. TNG-Cluster returns a variety of radio relics with diverse
morphologies, encompassing textbook examples of double radio relics, single
relics, and ``inverted" radio relics that are convex to the cluster center.
Moreover, the simulated radio relics reproduce both the abundance and
statistical relations of observed relics. We find that extremely large radio
relics ($>$ 2 Mpc) are predominantly produced in massive cluster mergers with
$M_{\rm 500c}\gtrsim8\times10^{14}~\rm~M_{\odot}$. This underscores the
significance of simulating massive mergers to study giant radio relics similar
to those found in observations. We release a library of radio relics from the
TNG-Cluster simulation, which will serve as a crucial reference for upcoming
next-generation surveys.
