  We present ALMA deep observations of the [CII] 158 $\mu$m emission line and
the continuum at 253 GHz and 99 GHz towards SDSS J0100+2802 at $z\simeq 6.3$,
the most luminous quasi-stellar object (QSO) at z$>$6. J0100+2802 belongs to
the HYPERION sample of luminous QSOs at $z\sim 6-7.5$. The observations (at
2.2" resolution in Band 3 and 0.9" resolution in Band 6) are optimized to
detect extended emission around the QSO. We detect an interacting, tidally
disrupted companion both in [CII], peaking at $z\sim 6.332$, and in continuum,
stretching on scales up to 20 kpc from the quasar, with a knotty morphology.
The higher velocity dispersion in the direction of the companion emission and
the complex morphology of tidally stretched galaxy suggest a possible ongoing
or future merger. For the newly-detected companion we derive a range of dust
mass, $M_{\rm dust}=(0.3-2.6)\times 10^7\ \rm M_\odot$, and of star formation
rate, SFR$=[35-344]\ \rm M_\odot$. This shows that both the QSO and its
companion are gas rich and that a major merger may be at the origin of the
boosted star formation. We also detect a broad blueshifted component in the
[CII] spectrum, that we interpret as a gaseous outflow for which we estimate a
mass outflow rate in the range $\dot{M}_{\rm out}=(118-269)\ \rm M_\odot\
yr^{-1}$. J0100+2802 was recently found to reside in a strong overdensity,
however this close companion remained undetected by both previous higher
resolution ALMA observations and by JWST-NIRCAM imaging. Our results highlight
the importance of deep medium-resolution ALMA observations for the study of
QSOs and their environment at the Epoch of Reionization.
