  We report the discovery of a $75\deg$ long stellar stream in Gaia DR2
catalog, found using the new STREAMFINDER algorithm. The structure is probably
the remnant of a now fully disrupted globular cluster, lies $\approx 3.8 \,
{\rm kpc}$ away from the Sun in the direction of the Galactic bulge, and
possesses highly retrograde motion. We find that the system orbits close to the
Galactic plane at Galactocentric distances between $4.9$ and $19.8 \, {\rm
kpc}$. The discovery of this extended and extremely low surface brightness
stream ($\Sigma_G\sim 34.3 \, {\rm mag \, arcsec^{-2}}$) with a mass of only
$2580\pm140 \, {\rm\,M_\odot}$, demonstrates the power of the STREAMFINDER
algorithm to detect even very nearby and ultra-faint structures. Due to its
proximity and length we expect that Phlegethon will be a very useful probe of
the Galactic acceleration field.
