  We present the first detailed analysis of the East Cloud, a highly disrupted
diffuse stellar substructure in the outer halo of M31. The core of the
substructure lies at a projected distance of $\sim100$ kpc from the centre of
M31 in the outer halo, with possible extensions reaching right into the inner
halo. Using Pan-Andromeda Archaeological Survey photometry of red giant branch
stars, we measure the distance, metallicity and brightness of the cloud. Using
Hubble Space Telescope data, we independently measure the distance and
metallicity to the two globular clusters coincident with the East Cloud core,
PA-57 and PA-58, and find their distances to be consistent with the cloud. Four
further globular clusters coincident with the substructure extensions are
identified as potentially associated. Combining the analyses, we determine a
distance to the cloud of $814^{+20}_{-9}$ kpc, a metallicity of $[Fe/H] =
-1.2\pm0.1$, and a brightness of $M_V = -10.7\pm0.4$ mag. Even allowing for the
inclusion of the potential extensions, this accounts for less than $20$ per
cent of the progenitor luminosity implied by the luminosity-metallicity
relation. Using the updated techniques developed for this analysis, we also
refine our estimates of the distance and brightness of the South-West Cloud, a
separate substructure analyzed in the previous work in this series.
