  Radial velocity (RV) surveys have discovered giant exoplanets on au-scale
orbits with a broad distribution of eccentricities. Those with the most
eccentric orbits are valuable laboratories for testing theories of high
eccentricity migration. However, few such exoplanets transit their host stars
thus removing the ability to apply constraints on formation from their bulk
internal compositions. We report the discovery of Kepler-1704 b, a transiting
4.15 $M_{\rm J}$ giant planet on a 988.88 day orbit with the extreme
eccentricity of $0.921^{+0.010}_{-0.015}$. Our decade-long RV baseline from the
Keck I telescope allows us to measure the orbit and bulk heavy element
composition of Kepler-1704 b and place limits on the existence of undiscovered
companions. Kepler-1704 b is a failed hot Jupiter that was likely excited to
high eccentricity by scattering events that possibly began during its gas
accretion phase. Its final periastron distance was too large to allow for tidal
circularization, so now it orbits it host from distances spanning 0.16 - 3.9
au. The maximum difference in planetary equilibrium temperature resulting from
this elongated orbit is over 700 K. A simulation of the thermal phase curve of
Kepler-1704 b during periastron passage demonstrates that it is a remarkable
target for atmospheric characterization from the James Webb Space Telescope,
which could potentially also measure the planet's rotational period as the hot
spot from periastron rotates in and out of view. Continued characterization of
the Kepler-1704 system promises to refine theories explaining the formation of
hot Jupiters and cool giant planets like those in the solar system.
