A faction similar to Mordor.
To be honest, I wouldn't really say there is a faction that works like that in DragonDawn. I guess the closest would be Ashapur as they start off quite large and have quite a few lighter troops they can build up pretty quickly. But then they also have a number of more expensive elite troops they can build which should stop them just spamming out huge stacks of lower tier units. The same could be said for other factions like the Empire or the Feds though really.
Lore wise, the Naugiri are a kind of 'zerg rush' sort of faction as they effectively moved an entire continent to try and wipe out pretty much all of the races except the other Elves. But they generally use bigger and better troops than the sort you'd normally zerg with so I guess they're not really a Mordor kind of faction. Then again, they are called Orcs as well...
Hi. I think probably the CHitkinnen (rats) are the closest to a swarm type bunch. They gather in spawns and then unleash assaults towards certain nations. They are in the minimod, but only playable in the main, bigger version. The Ghaurchlai in the West may be more like them too...again main mod. The Naugiri are relentless too, but they are a little more varied, and have some nasty big stuff. Other factions have units that are simply bred to overrun enemies. Feds have Penal Battalions, which have no chance of routing, and some others.
The CHitkinnen have some bigger rats; more powerful and much nastier, but they are 'homeless' and need to attack and conquer or just die. They have artillery of sorts too (Rabian Firethrowers), so they're a little more sophisticated despite being driven by hate and cowardice.