  Modern Galactic surveys have revealed an ancient merger that dominates the
stellar halo of our Galaxy (\textit{Gaia}-Sausage-Enceladus, GSE). Using
chemical abundances and kinematics from the H3 Survey, we identify 5559 halo
stars from this merger in the radial range $r_{\text{Gal}}=6-60\text{ kpc}$. We
forward model the full selection function of H3 to infer the density profile of
this accreted component of the stellar halo. We consider a general ellipsoid
with principal axes allowed to rotate with respect to the Galactocentric axes,
coupled with a multiply-broken power law. The best-fit model is a triaxial
ellipsoid (axes ratios 10:8:7) tilted $25^\circ$ above the Galactic plane
towards the Sun and a doubly-broken power law with breaking radii at 12 kpc and
28 kpc. This result resolves the long-standing dichotomy in literature values
of the halo breaking radius, being at either $\sim15\text{ kpc}$ or
$\sim30\text{ kpc}$ assuming a singly-broken power law. N-body simulations
suggest that the breaking radii are connected to apocenter pile-ups of stellar
orbits, and so the observed double-break provides new insight into the initial
conditions and evolution of the GSE merger. Furthermore, the tilt and
triaxiality of the stellar halo could imply that a fraction of the underlying
dark matter halo is also tilted and triaxial. This has important implications
for dynamical mass modeling of the Galaxy as well as direct dark matter
detection experiments.
