  Context. QSO B1420+326 is a blazar classified as a Flat Spectrum Radio Quasar
(FSRQ). In the beginning of 2020 it underwent an enhanced flux state. An
extensive multiwavelength campaign allowed us to trace the evolution of the
flare. Aims. We search for VHE gamma-ray emission from QSO B1420+326 during
this flaring state. We aim to characterize and model the broadband emission of
the source over different phases of the flare. Methods. The source was observed
with a number of instruments in radio, near infrared, optical (including
polarimetry and spectroscopy), ultra-violet, X-ray and gamma-ray bands. We use
dedicated optical spectroscopy results to estimate the accretion disk and the
dust torus luminosity. We perform spectral energy distribution modeling in the
framework of combined Synchrotron-Self-Compton and External Compton scenario in
which the electron energy distribution is partially determined from
acceleration and cooling processes. Results. During the enhanced state the flux
of both SED components drastically increased and the peaks were shifted to
higher energies. Follow up observations with the MAGIC telescopes led to the
detection of very-high-energy gamma-ray emission from this source, making it
one of only a handful of FSRQs known in this energy range. Modeling allows us
to constrain the evolution of the magnetic field and electron energy
distribution in the emission region. The gamma-ray flare was accompanied by a
rotation of the optical polarization vector during a low polarization state.
Also, a new, superluminal radio knot contemporaneously appeared in the radio
image of the jet. The optical spectroscopy shows a prominent FeII bump with
flux evolving together with the continuum emission and a MgII line with varying
equivalent width.
