All hail the mighty
That arising from the deep
…But Necrofex Collosi thou

For context:
Even though the Beastmen are generally a "bad" faction (they were the first new DLC faction way back in 2016, so overall quite lackluster even then and they have NEVER received an update so every other faction has outgrown them by a wide margin), they have a very specific trait: their mages can lower morale for all enemies on the battlefield just by being there.

So if you create an army with NOTHING but these mages, then the stacking morale debuff will be so extreme, all units that aren't absolute elite or "unbreakable" (as in: will always fight to the death) will pretty much run away INSTANTLY, the moment they enter the battlefield. For all intents and purposes, in Total War games morale is the TRUE health of your units.

And for those that don't: you still got your mages. While bad in Melee their magic can still be quite strong.