  We present photometry and spectroscopy of PS1-14bj, a hydrogen-poor
superluminous supernova (SLSN) at redshift $z=0.5215$ discovered in the last
months of the Pan-STARRS1 Medium Deep Survey. PS1-14bj stands out by its
extremely slow evolution, with an observed rise of $\gtrsim 125$ rest-frame
days, and exponential decline out to $\sim 250$ days past peak at a measured
rate of $0.01~{\rm mag~day}^{-1}$, consistent with fully-trapped $^{56}$Co
decay. This is the longest rise time measured in a SLSN to date, and the first
SLSN to show a rise time consistent with pair-instability supernova (PISN)
models. Compared to other slowly-evolving SLSNe, it is spectroscopically
similar to the prototype SN2007bi at maximum light, though lower in luminosity
($L_{\rm peak} \simeq 4.6 \times 10^{43} {\rm erg s}^{-1}$) and with a flatter
peak than previous events. PS1-14bj shows a number of peculiar properties,
including a near-constant color temperature for $>200$ days past peak, and
strong emission lines from [O III] $\lambda$5007 and [O III] $\lambda$4363 with
a velocity width of $\sim$3400 km/s, in its late-time spectra. These both
suggest there is a sustained source of heating over very long timescales, and
are incompatible with a simple $^{56}$Ni-powered/PISN interpretation. A
modified magnetar model including emission leakage at late times can reproduce
the light curve, in which case the blue continuum and [O III] features are
interpreted as material heated and ionized by the inner pulsar wind nebula
becoming visible at late times. Alternatively, the late-time heating could be
due to interaction with a shell of H-poor circumstellar material.
