Lord Pendragon said:
Say the party fights a dragon, and after a terribly difficult battle in which 2 PCs die, they manage to beat it down to its last bit of hp. Realizing it's almost dead, the dragon flies off and quickly makes its escape. Then, since it was only testing out the area to see how easy the prey might be, it decides to never come back. Are you saying you'd award the party no xp?
It doesn't matter if you actually kill something. Defeating the encounter is good enough, and when the lich dies, the encounter is over. It's CR has nothing to do with its immortality. That's just a nice little bonus that makes them good recurring villains.
Excuse me -
that depends entirely on what the PCs' objective is - if it is "overcome the dragon and free the area/kingdom/princess " they would get certainly XP for driving it off.
BUT if their mission's objective is destroy the dragon and end its menace to peaceful folks forever.... than well, sorry, no XP for driving the dragon off (although reduced XP for a tactical victory or and even full XP other monsters/adversaries overcome to get to the dragon would apply ). If they came to slay it, defeat it once and for all they must. How they accomplish that is open to inventiveness - kill it, entomb it alive, banish/transport it to some other place without appearant/obvious recourse, polymorph it into a mouse, change its alignment to lawful good.... whatever floats and has no apperant logical holes. But driving it off, or only delaying its plans for a few days won't do - hence no XP for "mission accomplished"
And, lets face it, there is a bunch of monster which have to be put down in a specific way to be destroyed - starting with the infamous Tarrasque (wish anyone ?) , several undead, many outsiders (who need to be destroyed on their homeplane) , also many creatures with regenerative abilities. That's part of the fun and challenge. If players and/or their characters don' t know, tough luck. And yes, Liches aren't easy to destroy mainly due to that. Otherwise, to a prepared party, they can fall astonishingly fast.
Speaking from personal experience, the PCs IMC faced just that problem some sessions back. They had battered the Lich to smithereens, but were well aware that he had retreated to his phylactery, which, right then, they couldn't spare the time to locate , so they knew he would be back.
None of them even asked for the XP for him for fun - they just knew he hadn't been dealt with (their mission was to secure a ruined area and make it safe for civillian use, which definitely included taking care of the Lich who claimed the area as a hereditary fief, and was pretty aggressive about it) yet. They met him a couple of sessions later, this time prepared for tracking him down, and took him out, with glee. XP awards for the first fight (roughly a CR10 instead of a CR15) were purely for tactical skill and secondary monsters, roleplaying and heroics, so that may have softened the blow over "delayed" XP some.
It's the same rule as overcoming a monster by guile, smarts or stealth without destroying it, just applied in reverse. If the characters can accomplish a mission without bloodshed and gain the full XP for it if they have the appropriate goals, then it must also be the case, that until the characters have permanently dealt with an adversary they need to destroy/take out of the equation for good, they cannot take it easy and reap the rewards, unless they go the whole nine yards.
And yes, if the villain returns through some means not inherently part of his class/type/template or equipment (say being ressurected ), then that is an entirely new challenge, with corresponding XP rewards.
YMMV.