If you want a challenge, you can try to recreate http://left4dead.wikia.com/wiki/The_Witch in Oblivion. Basically is a type of zombie that is passive (i.e, doesn't attack the player), unless you get too close to or hit her, which is called to "startle" her.
If you're going to do that, please make it so you can toggle all the special zombies in the game mode because sometimes I (or some one else) might not want them
Once startled, she runs (fast!) to the player and kills him in one hit. You'll need to script some sort of "attack-on-proximity if you're close to her more than 2/3 seconds" (as "attack on hit" on passive NPC's is already avaliable), and some sort of script like "GetPlayerHealth ---> SetNPCMeleeAttack", in order to assure one-hit kills. (P.D: I'm a noob at scripting, and I've invented the names, but I think you understand me
).
Only cons I see is that she must be placed mostly indoors, or inside cities, as it'd be too easy to avoid her in the outdoors.

Only cons I see is that she must be placed mostly indoors, or inside cities, as it'd be too easy to avoid her in the outdoors.
Or you could make the hit effect something like drain health 1000000 and damage health 1000000