  We present magnetohydrodynamic simulation of the evolution from
quasi-equilibrium to onset of eruption of a twisted, prominence-forming coronal
magnetic flux rope underlying a corona streamer. The flux rope is built up by
an imposed flux emergence at the lower boundary. During the quasi-static phase
of the evolution, we find the formation of a prominence-cavity system with
qualitative features resembling observations, as shown by the synthetic SDO/AIA
EUV images with the flux rope observed above the limb viewed nearly along its
axis. The cavity contains substructures including ``U''-shaped or horn-liked
features extending from the prominence enclosing a central ``cavity'' on top of
the prominence. The prominence condensations form in the dips of the highly
twisted field lines due to runaway radiative cooling and the cavity is formed
by the density depleted portions of the prominence-carrying field lines
extending up from the dips. The prominence ``horns'' are threaded by twisted
field lines containing shallow dips, where the prominence condensations have
evaporated to coronal temperatures. The central ``cavity'' enclosed by the
horns is found to correspond to a central hot and dense core containing twisted
field lines that do not have dips. The flux rope eventually erupts as its
central part rises quasi-statically to a critical height consistent with the
onset of the torus instability. The erupting flux rope accelerates to a fast
speed of nearly 900 km/s and the associated prominence eruption shows
significant rotational motion and a kinked morphology.
