As per Darkening, the three that are sent are crazy tough, but it's possible for a party to fight one off (not kill, or permanently defeat, of course). The book states that they aren't at their full strength, as their lord is still regaining his power. By the time of the Fellowship of the Ring, they will be much stronger.Woodclaw wrote: Plus I think that giving Khamul a Morgul Blade will simply be overkill. He's already a opponent behind the abilities and powers of most PCs and many NPCs roaming the Wilderlands. As far as I can tell only Radagast and, possibly, Thranduil might be able to face him in direct combat.
That said, they cause a world of pain before even bringing a weapon to bear, and even a party that defeats one will likely be permanently affected.