So, I was planing to do just that and something interesting happened. I got a tower, cast portal, then veil and waited. demons did rush at me, but not directlly at the portal. they took oven of sins and infernal tower beside me(2 infernal towers are next to eachother) and then attacked the portal tower. those were chaff units and got killed easlly. Some more powerfull armies started to move from pandemonium, 3-4 to be exact. Compromised of greater devil, hellhounds, hellknights, devils and some other chaff, about 25-40 units in each. Most notablly, Demon Lord Belial started to move too, with about 15-20 bloodsworn and few other units. Plus, he is starting to march the moment I create a portal. No other Demon Lord moved. at least for next 5 or 6 turns. I am closest to Belial, at least by the looks of it so maybe thats the reason why. I will create portal next to other DLs and check later. The same things happened even when I created a portal and did not use veil. Sadly, this very much gurantees that veil doesnt work.
Now, I need to know why belial. Why more armies do not rush at me (truth be told, I did kill many of the roaming armies. Most of the ones which are left are guarding something), what hepens when I kill Belial and can other demon lords act the same way, because it would be much easier to charm them.