M-Plate is an amalgamated being composed of Emplate and the M Twins.
Although, the M Twins had already amalgamated into M, so that's what any mentions on another website or this one of M-Plate being "Emplate and M fused into one" mean.
Having been made from so many powerful mutants, M-Plate's power was thought to be unsurpassed, while M-Plate was alive. Take note, however, that gestalting/amalgamating powerful mutants together doesn't necessarily create a more powerful amalgamation (the exact power and personality traits are more or less random). However, in this case, that appears to have been the case, so M-Plate was too powerful for Generation X to handle on its own.
As to what M-Plate's powers are, they are described by M-Plate as being able to "command the flux of inter-dimensional reality itself" and being able to distort such at will (in Generation X #35). If you understand what that means, you understand M-Plate's powers. M-Plate has also been shown to be able to fire off psionic blasts, create force fields that require concentration to maintain, bat people away with superhuman strength, fire from its hand what it calls "real fireworks" at Jubilee, and kill Chimera simply by firing beams at her through its eyes.
M-Plate's ultimate goal was to merge all consciousness in the universe into one universally aware being (namely M-Plate itself). M-Plate tried to get Synch to synch up with the resistant Gaia to activate the Universal Amalgamator to no avail.
Once M-Plate had captured members of Generation X, slowly contracting a force field around them to kill them, Synch decided he would do as M-Plate desired and use Gaia's ability to activate the Universal Amalgamator; he felt he had no other recourse because he couldn't stand to watch his friends die. Luckily, however, a certain rat stepped in to save the day.
Because of the kindness shown to him by Generation X, Hollow in particular, a rodent-mutant named Dirt Nap heroically rushed in and got the drop on M-Plate. Dirt-Nap swallowed M-Plate whole and subsequently exploded, separating out M-Plate into its representative St. Croix siblings. It's known information that explosions are what tends to disrupt a gestalted St. Croix into its parts.