  We report the discovery of a metamagnetic phase transition in a polar
antiferromagnet Ni$_3$TeO$_6$ that occurs at 52 T. The new phase transition
accompanies a colossal magnetoelectric effect, with a magnetic-field-induced
polarization change of 0.3 $\mu$C/cm$^2$, a value that is 4 times larger than
for the spin-flop transition at 9 T in the same material, and also comparable
to the largest magnetically-induced polarization changes observed to date. Via
density-functional calculations we construct a full microscopic model that
describes the data. We model the spin structures in all fields and clarify the
physics behind the 52 T transition. The high-field transition involves a
competition between multiple different exchange interactions which drives the
polarization change through the exchange-striction mechanism. The resultant
spin structure is rather counter-intuitive and complex, thus providing new
insights on design principles for materials with strong magnetoelectric
coupling.
