Correct. In one of my games a hero was guarding the portal as well as a normal stack. It is tricky and did require a couple of reloads to achieve, but I managed to get around them and into Demonis. The important thing is you can't allow the enemy to be too close behind you otherwise you won't be able to get back into Underground Sea without fighting. So it is important to save the game, try to come back and see if it's possible, then reload in Demonis and continue.
It's not that hard, KBH. You just suck.
