  Spectroscopic observations of prominence eruptions associated with coronal
mass ejections (CMEs), although relatively rare, can provide valuable plasma
and 3D geometry diagnostics. We report the first observations by the Interface
Region Imaging Spectrograph (IRIS) mission of a spectacular fast CME/prominence
eruption associated with an equivalent X1.6 flare on 2014 May 9. The maximum
plane-of-sky and Doppler velocities of the eruption are 1200 and 460 km/s,
respectively. There are two eruption components separated by ~200 km/s in
Doppler velocity: a primary, bright component and a secondary, faint component,
suggesting a hollow, rather than solid, cone-shaped distribution of material.
The eruption involves a left-handed helical structure undergoing
counter-clockwise (viewed top-down) unwinding motion. There is a temporal
evolution from upward eruption to downward fallback with less-than-free-fall
speeds and decreasing nonthermal line widths. We find a wide range of Mg II k/h
line intensity ratios (less than ~2 expected for optically-thin thermal
emission): the lowest ever-reported median value of 1.17 found in the fallback
material and a comparably high value of 1.63 in nearby coronal rain and
intermediate values of 1.53 and 1.41 in the two eruption components. The
fallback material exhibits a strong ($> 5 \sigma$) linear correlation between
the k/h ratio and the Doppler velocity as well as the line intensity. We
demonstrate that Doppler dimming of scattered chromospheric emission by the
erupted material can potentially explain such characteristics.
