Selling their soul after death isn't going to be much of a price for the players; for the characters definitely, but not players. You should make the price definitely be things they have to pay during the campaign. One thing that I've floated around in my mind is for pacts to serve as an alternate leveling method. As far as reasonable prices, questing would be a great idea. Depending on the exact nature of the fiends, sacrifices could be made. Another option is to play it very similarly to Wish / Miracle.
Selling their soul after death isn't going to be much of a price for the players; for the characters definitely, but not players. You should make the price definitely be things they have to pay during the campaign. One thing that I've floated around in my mind is for pacts to serve as an alternate leveling method. As far as reasonable prices, questing would be a great idea. Depending on the exact nature of the fiends, sacrifices could be made. Another option is to play it very similarly to Wish / Miracle.
It will matter if selling their soul means no raise dead or resurrection will work for them, because their soul is already spoken for immediately after death. For the rest of their brief lives only breath of life will bring them back from the brink before they are finally and irrevocably deceased, and that requires a caster to be both quick and close (and willing to risk a hit from whatever took out the damned pc). Such a campaign would require some additional rules on what happens to the soul immediately after death. Clearly there is a 1 rd delay so that breath of life works, but maybe the spirit lingers for a bit before the fiends grab hold of it, where raise dead and the like will work, but the contract holder will then see it as a breach and take action.
Selling their soul after death isn't going to be much of a price for the players; for the characters definitely, but not players. You should make the price definitely be things they have to pay during the campaign. One thing that I've floated around in my mind is for pacts to serve as an alternate leveling method. As far as reasonable prices, questing would be a great idea. Depending on the exact nature of the fiends, sacrifices could be made. Another option is to play it very similarly to Wish / Miracle.
EDIT: Ninja'd. Well here's a bit of detail for 3.5 at least.
It can be a cost. In 3.5 that means if their character ever dies they cannot be resurrected short of either appealing in hellish courts (Check the BVD they have appeal courts apparently) or hunting the fiend down and killing it.
So they may get a cool boost, but if they ever die in combat the player has to reroll.