Tinker Gnome
Adventurer
I like my PCs to be The Unchosen One so I generally do not use prophecies.
I've always wanted to try using a "20/20 Hindsight Prophecy"...
In which there is a prophecy, and perhaps the PCs know that there is a prophecy, but they don't necessarily know what the prophecy says. I'd let the PCs play through a large chunk of the campaign, and then reveal the prophecy to them... A prophecy that I'd have only just then created, and reverse engineered to match their exploits.
...I strongly recommend against this approach- you would basically be telling some pcs that because of their character generation choices, they get screwed.
Re: the Witch-King
Something that vague still has plenty of wiggle room, especially in a FRPG. It could mean death via woman, death via non-Human, death via an accident (falls off cliff), death via carelessness (his own magic misfires or is bounced back at him, the PCs convince his minions to redeem themselves and turn on him, a summoned creature kills him, there is divine intervention prayed for by a cleric...
It doesn't screw the PCs unless you, the DM, have ONLY ONE meaning to the prophecy. Believe me, creative players will think of SOMETHING that satisfies the prophesy...you just have to decide it's good enough.