Ive statted it up a bit, and with between 600 and 700 wizards. They will all have something similar to the red wizards circle feats. They also have a magic item that creates a portal (has anyone read the Riftwar saga by Raymond E Feist?), however, it needs casters continually working with it to keep it open.
As for the reason, these people are similar to locusts. They move into an area, eat it bare (using the few slaves they capture to harvest everything), then move to another area. They destroy everything. Once they move into an area, they leave nothing behind. Their creed is destruction. If nothing stops them....well, lets just say Faerun will end up looking like the Anauroch desert. They kill all adults who stand up to them. Those who surrender are made into slaves. Children under 5 are adopted into the Horde and raised to become new members. Those over five who can work are made to. Those who cant......you get the picture. They dont care for anyone but themselves. The Horde started out on one world, after it had destroyed everything, the wizards developed a machine to create portals to other worlds. That was a thousand years ago. Now, none of the wizards understand how it was made. They just know how to work it. The PCs will be charged with finding a way to destroy it. Hopefully with some of the Horde on the other side still.
The Horde is split up into military groups, similar to the Roman military. 80 men to a century, 6 centuries to a cohort, and 10 cohorts to a legion. 10 legions comprise an army. Each army is named after a predator, with the legions using numbers. so you might have III Wolf, IV Lion, and I Leopard as three different legions. Each century is controlled by a centurion, each cohort by a High centurion, and each legion is controlled by a Legate. The one thing they DONT have that the Roman legions had was a tight military formation. They dont use standard equipment, instead picking whatever they like. However, they do understand how to put melee fighters in front, with those with reach weapons 5 foot behind, and ranged fighters behind that. The first cohort in each legion has only 5 centuries, with about 500 or so experts with them....engineers, blacksmiths, cooks, etc. These are the people who fix the weapons and armor, take care of meals, and make the siege engines. Each legion also contains 100 dwarven miners/sappers.