But for the creation of Trollocs, Mydraal, Draghkar, Gholem, etc., the shadow's forces would consist of nothing but a few renegade Aes Sedai from the age of legends.

What makes the shadow so threatening are the millions of trollocs and other shadowspawn created by Aginor. I think Aginor's creations have killed more people than all of the other forsaken combined.

Semirhage also did quite a bit of damage in seanchan. Looking at the maps of the Big White Book, to put that entire continent into a civil war that might never end for another thousand years is a pretty big accomplishment. Because of Semirhage, literally millions of Seanchan will be too busy fighting each other instead of preparing for the last battle.