In the end of the "blow the bridge" mission two portals appeared: The usual iron-maiden-like Mordessa's portal and one huge shapeless black one which originally I thought it was a graphical bug.

Of course, when both opened, and given the hints the narrator let drop before the last mission, I assumed the "glitch" was a portal made by the big bad guy himself, so I thought it could be a good chance for a promotion.
Of course, the bad guy himself asked me to kill Mordessa. The american dream: Kill your boss, get a promotion.

But I'm kinda of completist, and I'm wondering if I had chosen the Mordessa's portal, a different "final" mission would have appeared. Is there such a mission? After having chosen the "promotion" to big bad guy's hitman -and having completed that alleged final mission- there's a way to unlock the alleged final Mordessa's mission besides starting a new evil campaign?

Its not I would do it, I'm having a surprising load of fun with the game (besides the nightcrawlers. I hate them), but hey, just to save some time for other games :V

If I remember correctly, you get a cutscene of Mordessa killing you and then a "you died" screen. So yeah, no extra mission. =\

Grilledfish is correct, there's just a short cutscene if you enter Mordessa's portal. If you wanted to see it, you can still replay that level after completing the game and take the other exit. ;)

Uh, good to know it, thanks!

Seems in the evil lands of Enclave the employment promotions are quite... drastic.

Hope I don't get fired :S