  The hour-long, gradual phase of solar flares is well-observed across the
electromagnetic spectrum, demonstrating many multi-phase aspects, where cold
condensations form within the heated post-flare system, but a complete
three-dimensional (3D) model is lacking. Using a state-of-the-art 3D
magnetohydrodynamic simulation, we identify the key role played by the Lorentz
force through the entire flare lifespan, and show that slow variations in the
post-flare magnetic field achieve the bulk of the energy release. Synthetic
images in multiple passbands closely match flare observations, and we quantify
the role of conductive, radiative and Lorentz force work contributions from
flare onset to decay. This highlights how the non-force-free nature of the
magnetic topology is crucial to trigger Rayleigh-Taylor dynamics, observed as
waving coronal rays in extreme ultraviolet observations. Our C-class solar
flare reproduces multi-phase aspects such as post-flare coronal rain. In
agreement with observations, we find strands of cooler plasma forming
spontaneously by catastrophic cooling, leading to cool plasma draining down the
post-flare loops. As there is force balance between magnetic pressure and
tension and the plasma pressure in gradual-phase flare loops, this has
potential for coronal seismology to decipher the magnetic field strength
variation from observations.
