Give the characters the ability to be retired with dignity. Give them a goal; you want your keep and lands, maybe a lordship? The party members maybe want to open a joint business? Maybe a bar in some huge city or in remote area? Maybe on another plane who knows? Maybe the party members want to open a business each the fighter wants to open a war college? The wizard a wizard school, the psion a psi temple? Have them find a perfect town...
Point is make this a goal and then show them some obvious problems in there way. This town already has a recently appointed new govt....several other adventurers but they are evil and milking the whole town for everthing its worth. They may encounter the villian subs 1 at a time then the villians get wise and strike party with ruthless efficiency. Make them classes/character types, my favorite for high level campaigns is pit the party against a crafty int enemy –A evil party of adventurers almost equaling the party’s if not outright rivaling! Make them tough with magicals that they know how to use, they should have some special abilities/magicals to confuse party.
Mix up the classes An Assassin/Shadow dancer, have this guy kill the paladins horse, have him then target the Wizards familiar, the druids animal companion. Make him a real SOB! Every 3 rounds he will attack and kill 1 of the party’s subordinates, or at least try to, make him specialize in gorilla warfare, stay out of sight yet he sees inviso. etc. This will get your party excited very quickly!!!! This is his fav tactic because it weakens the party and doesnt endanger him as much, or so his theory claims.
A Cerebramancer at 15th level will cast as a 12th level wizard and manifest as a 12th level Psion. Pick his powers & feats intelligently and couple them with his spells and you have a very potent stand back and fry the enemy kinda guy........hint maximized mind thrust vs warrior types & have him sick his astral constructs on the party cleric & Wizards....to piss off the party rogue have him sick an astral construct with a 29 or more str, that grapples. very very powerful. He should be covered in tattoos and a couple of contingency spells/contigency via contigency feat just to insure a get away if needed...he will escape and be healed and return with something even worse for the party almost immediately!!!
With these two main bad guys just add on a couple of meat heads with really really high AC's, make the party happy when they hit, have the meat heads beefed with stoneskin etc. Sorlyns floating disc gives a +10 AC from ? book but it was a 3rd level spell. Give them great buffs to saves, immunities to a couple of energy types. There to hit and damage mayb e slightly lowered to compensate for such a high defense
Make the Mind control guy a Vampire lord? Very very powerfull and he looks normal so the party will never know he is a vamp! He will have several vamps he summons to attack in his rather potent darkness spell......darkness from some evil powerfull magic item something the party cant over come or dispel...
Thorncrest
Give the characters the ability to be retired with dignety. But make em pay dearly!!!