  We report the discovery and dynamical analysis of 2015 BP$_{519}$, an extreme
Trans-Neptunian Object detected detected by the Dark Energy Survey at a
heliocentric distance of 55 AU and absolute magnitude Hr= 4.3. The current
orbit, determined from a 1110-day observational arc, has semi-major axis
$a\approx$ 450 AU, eccentricity $e\approx$ 0.92 and inclination $i\approx$ 54
degrees. With these orbital elements, 2015 BP$_{519}$ is the most extreme TNO
discovered to date, as quantified by the reduced Kozai action, which is is a
conserved quantity at fixed semi-major axis $a$ for axisymmetric perturbations.
We discuss the orbital stability and evolution of this object in the context of
the known Solar System, and find that 2015 BP$_{519}$ displays rich dynamical
behavior, including rapid diffusion in semi-major axis and more constrained
variations in eccentricity and inclination. We also consider the long term
orbital stability and evolutionary behavior within the context of the Planet
Nine Hypothesis, and find that BP$_{519}$ adds to the circumstantial evidence
for the existence of this proposed new member of the Solar System, as it would
represent the first member of the population of high-i, $\varpi$-shepherded
TNOs.
