We report the discovery and dynamical analysis of 2015 BP _ { 519 } , an extreme Trans-Neptunian Object detected by the Dark Energy Survey at a heliocentric distance of 55 AU , perihelion of \sim 36 AU , and absolute magnitude H _ { r } = 4.3 . The current orbit , determined from an 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 , \eta _ { 0 } = ( 1 - e ^ { 2 } ) ^ { 1 / 2 } \cos { i } , which is a conserved quantity at fixed semi-major axis a for axisymmetric perturbations . We discuss the orbital stability and evolution of this object , and find that under the influence of the four known giant planets 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 2015 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 .