So looking at a possible army based on stealth and being sneaky. Fluff wise I wanted an army that would be preying on isolated towns and what not, moving in during the dead of night and leaving nothing but a ghost town in their wake. Mostly to show terror and confusion in the enemy. I guess I just wanted to use as little orc/uruks in an evil army and a fluffy army that may still win games.

Leader
120 The betrayer
54 Watcher of Karna x6
60 Watcher of Karna Bows x6

Warband #2
90 Hasharin
42 Haradrim Warriors bows x6
42 Haradrim Warriors spears x6

Warband #3 (Mordor)
120 The Tainted
45 Dead Marsh Specters x3
120 Morgul Stalkers x8

total 693

So as you can see I went with bow heavy over horses as animals tend to not understand stealth, especially a horse at full gallop. Things like the Hasharin and Stalkers are obvious as their fluff matches well. The Betrayer and Watchers hitting on a 3+ makes them re-roll any misses. (can't add more bows with the mordor allies, unless I am doing this wrong). The tainted and specters I put in because they as that "bump in the night" feel to the army.

Wow you have come up with a very interesting theme. I am not the one to tell you if your army is a tactical or efficient one, but have you thought of adding a Shade or Barrow-Wights? They seem to me to add to the feeling your army wants to have on poor villagers.

Put the Watchers with bow in the second warband so they can have spear support, it really helps. Nice army it's really cool, I think I'll use it sometime... By the way the poison is for wounding not hitting

The numbers are a little low, i think a few lucky hits on will break your army and ruin it, have you thought about makign it an elite army? I'd drop all the harad soldiers (maybe morgul stalkers too) for Dalamyr and Shelob. Shelob is MUST for stealth & assassination, she can also function as a bow proof tank. Maybe get some arbalesters too simply as cover for the assassins.