Doom brigade.

[wuhan]

I was going to have the PC's fight a bunch of low level clerics...

But what happens if all 5 clerics cast doom on the barbarian?

Does it stack?

[x9ss]

No. Spells with the same name do not stack. (Other than some specified exceptions.)

[B0sh1]

You need to ask what happens if they all start channeling negative energy and could somehow exclude some of their allies with a feat =P

[wuhan]

ok, but cause fear, is a separate effect right? And that stacks with doom?

[x9ss]

I don't believe it's possible to be both Shaken and Frightened at once. Thus, if the Barbarian failed saves against each spell, he would end up Frightened for 1d4 rounds (as per the Cause Fear spell) and then Shaken for the remaining duration of Doom (1 min/level).