Ok, watched the episode. Apparently Tirek was trapped in Tartarus since before the founding of Equestria. He escaped last season when Cerberus left his place and has been biding his time growing in power ever since.
His magic stealing ability depends on the difference in power - those near his power he can only drain if he gets a clear shot and it is a full round action. - those way beneath he can drain en masse as an area effect. The draining leaves the victim with paled eyes and no cutie mark and takes away their signature abilities - unicorns lose magic, earth ponies lose strength and pegasi can no longer defy physics and become too heavy to fly.
As he drains more and more energy, Tirek gets closer to his generation 1 appearance, rejuvenating and bulking up. Once he has grown to huge size, he is powerful enough to drain the likes of Discord.
In battle, he is a brute - he leaps, charges and spams telekinesis and fire evocation (which, depending on his current power level ranges from "d4 damage ray of heat" to "turned Golden Oaks Library into a cloud of splinters")
There should be a good picture of him by tomorrow, perhaps this evening.
