  The link between the fate of the most massive stars and the resulting
supernova (SN) explosion is still a matter of debate, in major part because of
the ambiguity among light-curve powering mechanisms. When stars explode as SNe,
the light-curve luminosity is typically sustained by a central engine
(radioactive decay, magnetar spin-down, or fallback accretion). However, since
massive stars eject considerable amounts of material during their evolution,
there may be a significant contribution coming from interactions with the
previously ejected circumstellar medium (CSM). Reconstructing the progenitor
configuration at the time of explosion requires a detailed analysis of the
long-term photometric and spectroscopic evolution of the related transient. In
this paper, we present the results of our follow-up campaign of SN 2020faa.
Given the high luminosity and peculiar slow light curve, it is purported to
have a massive progenitor. We present the spectro-photometric dataset and
investigate different options to explain the unusual observed properties that
support this assumption. We computed the bolometric luminosity of the supernova
and the evolution of its temperature, radius, and expansion velocity. We also
fit the observed light curve with a multi-component model to infer information
on the progenitor and the explosion mechanism. Reasonable parameters are
inferred for SN 2020faa with a magnetar of energy Ep=1.5(+0.5,-0.2)x10^50 erg
and spin-down time t(spin)=15+/-1 d, a shell mass M(shell)=2.4(+0.5,-0.4) Msun
and kinetic energy Ekin(shell)=0.9(+0.5,-0.3)x 10^51 erg, and a core with
M(core)=21.5(+1.4,-0.7) Msun and Ekin(core)=3.9(+0.1,-0.4)x10^51 erg. In
addition, we need an extra source to power the luminosity of the second peak.
We find that hidden interaction with either a CSM disc or delayed, choked jets
is a viable mechanism for supplying the required energy to achieve this effect.
