  We present optical observations of supernova SN 2014C, which underwent an
unprecedented slow metamorphosis from H-poor type Ib to H-rich type IIn over
the course of one year. The observed spectroscopic evolution is consistent with
the supernova having exploded in a cavity before encountering a massive shell
of the progenitor star's stripped hydrogen envelope. Possible origins for the
circumstellar shell include a brief Wolf-Rayet fast wind phase that overtook a
slower red supergiant wind, eruptive ejection, or confinement of circumstellar
material by external influences of neighboring stars. An extended high velocity
Halpha absorption feature seen in near-maximum light spectra implies that the
progenitor star was not completely stripped of hydrogen at the time of core
collapse. Archival pre-explosion Subaru Telescope Suprime-Cam and Hubble Space
Telescope Wide Field Planetary Camera 2 images of the region obtained in 2009
show a coincident source that is most likely a compact massive star cluster in
NGC 7331 that hosted the progenitor system. By comparing the emission
properties of the source with stellar population models that incorporate
interacting binary stars we estimate the age of the host cluster to be 30 - 300
Myr, and favor ages closer to 30 Myr in light of relatively strong Halpha
emission. SN 2014C is the best-observed member of a class of core-collapse
supernovae that fill the gap between events that interact strongly with dense,
nearby environments immediately after explosion and those that never show signs
of interaction. Better understanding of the frequency and nature of this
intermediate population can contribute valuable information about the poorly
understood final stages of stellar evolution.

High Energy Astrophysical Phenomena;Solar and Stellar Astrophysics