  In this paper, we report the multiwavelength observations of an erupting
prominence and the associated CME on 13 May 2013. The event occurs behind the
western limb in the field of view of SDO/AIA. The prominence is supported by a
highly twisted magnetic flux rope and shows rapid rotation in the
counterclockwise direction during the rising motion. The rotation of the
prominence lasts for $\sim$47 minutes. The average period, angular speed, and
linear speed are $\sim$806 s, $\sim$0.46 rad min$^{-1}$, and $\sim$355 km
s$^{-1}$, respectively. The total twist angle reaches $\sim$7$\pi$, which is
considerably larger than the threshold for kink instability. Writhing motion
during 17:42$-$17:46 UT is clearly observed by SWAP in 174 {\AA} and EUVI on
board the behind STEREO spacecraft in 304 {\AA} after reaching an apparent
height of $\sim$405\,Mm. Therefore, the prominence eruption is most probably
triggered by kink instability. A pair of conjugate flare ribbons and post-flare
loops are created and observed by STA/EUVI. The onset time of writhing motion
is consistent with the commencement of the impulsive phase of the related
flare. The 3D morphology and positions of the associated CME are derived using
the graduated cylindrical shell (GCS) modeling. The kinetic evolution of the
reconstructed CME is divided into a slow-rise phase ($\sim$330 km s$^{-1}$) and
a fast-rise phase ($\sim$1005 km s$^{-1}$) by the writhing motion. The edge-on
angular width of the CME is a constant (60$^{\circ}$), while the face-on
angular width increases from 96$^{\circ}$ to 114$^{\circ}$, indicating a
lateral expansion. The latitude of the CME source region decreases slightly
from $\sim$18$^{\circ}$ to $\sim$13$^{\circ}$, implying an equatorward
deflection during propagation.
