I didn't have a lore planned, but it's not hard to make one either.

The 1st brigade was one of the most outstanding regiments during the era of conflicts, their victories were so many that other enemy regiments were afraid to face them, but one day, they faced the necrocorvus brigade, this regiment caused their enemies to contract diseases with pagan magic or throwing corpses towards the enemy position.
With an ambush they destroyed the 1st brigade, but their legacy still stands, many other brigades tried to obtain the achievements of the 1st brigade, failing in the process...