  We report the intriguing large-scale dynamic phenomena associated with the
M6.5 flare~(SOL2015-06-22T18:23) in NOAA active region 12371, observed by
RHESSI, Fermi, and the Atmospheric Image Assembly (AIA) and Magnetic Imager
(HMI) on the Solar Dynamic Observatory (SDO). The most interesting feature of
this event is a third ribbon (R3) arising in the decay phase, propagating along
a dimming channel (seen in EUV passbands) towards a neighboring sunspot. The
propagation of R3 occurs in the presence of hard X-ray footpoint emission, and
is broadly visible at temperatures from 0.6~MK to over 10~MK through the
Differential Emission Measure (DEM) analysis. The coronal loops then undergo an
apparent slipping motion following the same path of R3, after a $\sim$ 80~min
delay. To understand the underlying physics, we investigate the magnetic
configuration and the thermal structure of the flaring region. Our results are
in favor of a slipping-type reconnection followed by the thermodynamic
evolution of coronal loops. In comparison with those previously reported
slipping reconnection events, this one proceeds across a particularly long
distance ($\sim$60 Mm) over a long period of time ($\sim$50 min), and shows two
clearly distinguished phases: the propagation of the footpoint brightening
driven by nonthermal particle injection and the apparent slippage of loops
governed by plasma heating and subsequent cooling.
