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We use the $Gaia$ DR2 RR Lyrae sample to gain an uninterrupted view of theGalactic stellar halo. We dissect the available volume in slices parallel tothe Milky Way's disc to show that within $\sim30$ kpc from the Galactic centrethe halo is triaxial, with the longest axis misaligned by $\sim70^{\circ}$ withrespect to the Galactic $x$-axis. This anatomical procedure exposes two largediffuse over-densities aligned with the semi-major axis of the halo: theHercules-Aquila Cloud and the Virgo Over-density. We reveal the kinematics ofthe entire inner halo by mapping out the amplitudes and directions of the RRLyrae proper motions. These are then compared to simple models with differentanisotropies to demonstrate that the inner halo is dominated by stars on highlyeccentric orbits. We interpret the shape of the density and the kinematics ofthe $Gaia$ DR2 RR Lyrae as evidence in favour of a scenario in which the bulkof the halo was deposited in a single massive merger event.

The shape of the Galactic halo with $Gaia$ DR2 RR Lyrae. Anatomy of an ancient major merger