High Level Death
First, I must state for the record that due to horrible luck, I have not been able to run a campaign to a point where any player has made it beyond 10th level (that being in 2nd Edition).
Second, It think that as long as the DM is willing to be very very mean about it, you can kill damn near anything.
Character durablity can get quite impressive at the higher levels, but alot of that effectiveness is often the result of buffing spells cast before an anticipated fight.
Another point is that it is very uncommon for players beyond 5th level to fight an equal number of equal level opponents. Usually, its a large number of lower HP creatures or one or two tank like creatures.
Its one thing for the players to fight off an Ancient Dragon. Its another for them to fight off a Mated pair of ancient dragons and their offspring. Its yet another for the players to be attacked by those dragons in the open as opposed to fighting them in or near their lairs.
If you need something to really put up a fight against your players, you have several options.
1) Equal char level NPC's: Create 3 or 5 20th level opponents for your 20th level party.
2) Powerful Monsters: See the Epic Level Handbook, or some of the CR 20 beasts in the Call of Cthulhu book, or the avatars of gods from Cthulhu or Dieties and Demigods. Most D&D players wont recognize a Shoggoth, or have any idea of what to do about it.
3) Very Hostile Environments: Suddenly sending a Barbarian through a gate to the plane of elemental fire will probably do him in.
As for permanant death, in a game with a wish spell, nothing short of the DM can really make a death permanant. You can decide that a hostile god has trapped the PC's souls, or do other things to the players that make the PC's impossible to ressurect / rescue.
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