I would think that, with a full kingdom behind him, a king could cast pretty much any spell after a while. I rule that any spell I need cast in an area, a king or general underneath a king can get someone to do it...if you have a fighter who enjoys walking through prison walls, hide permanent prismatic walls in them. That's also how I guard doors I don't what the players just to run through.
I don't approach things in quite that way, CAFargo.
My paradigm is that most everyone in my DnD world is 3rd level or less. A few have achieved 4th level. And, some really legendary folks have achieved 5th level. Sixth level is "unthinkably high."

If I haven't mentioned, earlier in this thread, I'm a fan of Ryan Stoughton's E6 and of another paper on The Alexandrian called Callibrating Your Expectations (both are linked in my sig).
Most kings in the world I run wouldn't have levels in PC classes, at all. They might, at most, be third or fourth level nobles, perhaps gestalted with warrior levels. And, most wouldn't be able to cast any known spell through subordinates at a moments notice. Spellcasters are rare and their spells aren't usually at the beck and call of kings.
High level spell casters are a rare thing, too. Sixth level is considered "an unthinkably high level" (to quote Gygax from the AD&D DMG). And, spellcasters of such level may or may not aid such a king depending on their goals. And, it may be quite possible that the king isn't on good terms with such personages. Notice that I'm not framing this for the need of the king. King's don't command spellcasters. Anyone capable of bending the fabric of the universe to such a degree through magic isn't likely the lackey of another man. At best, the king might have good relations with one that might be able to help from time to time when said spell caster isn't furthering his or her own goals, elsewhere, away from the royal courts.
As permanency is a 5th level spell (usually requiring at least 9th level wizard to cast) and prismatic wall is an 8th level spell (usually requiring a 15th level caster), permanent prismatic walls haven't even been heard of in the world I DM. Spells of 4th level and above are the purview of the gods and are not at the beck and call of mortal kings or their servants. Second level spells are about as powerful as most any king could hope for without being on good terms with a spellcaster of considerable renown. Instead, it'd be much more likely that the king would have an expert specialized in alchemy and herbology.
I would say permanent Con damage, because a character heals temp damage at a rate of 1/day. This will automatically heal a character killed by a death affect, which would SUCK for the killer.
CON damage still kills if the constitution score is brought to zero in the RAW SRD, as far as I can tell. And, dead characters don't automatically gain constitution while losing hit points. That wouldn't make any sense, to me, at all.
Right now, I'm thinking of the resurrection scene in
Conan the Barbarian. Afterward, Conan is flexing and looking at his hand, perhaps pondering the answer to the Riddle of Steel as given to him by Thulsa Doom. But, it could also be something else, if we imagine these rules in play.
Conan was killed via crucifixion on the Tree of Woe. Now, if we imagine that crucifixion does CON damage, much like a well executed death attack from an assassin, then it's reasonable to conclude that when Conan was practicing a little with his sword and looking at his hand as he clenched it, that he was spending time not only thinking about his recent experiences, but he was regaining his constitution before continuing his mission of revenge.
The strange body paint applied by Akiro (spelling?) may have been a form of a gentle repose spell or ceremony that accomplished something similar?
In any case, either ability damage or ability drain would work. The difference is in whether the DM want the CON damage to heal naturally, if the character is brought back via lesser restoration (which can only restore 1d4 points on CON damage, but cannot restore CON drain). For me, this answers the question of whether or not it should be CON damage or drain. Since Restoration is a 4th level spell, it wouldn't be reasonable for me to require it in a world where 7th level casters are non-existent.
So, it comes down to the type of world that the DM is running. In my world, Constitution ability damage is fine. Especially, since it can only be cured through rest (if the victim survives the assassin's attack) or through lesser restoration from a rare priest capable of such.
Now, all that said, I realize the since the Assassin is a prestige class in the SRD that is probably only availble to characters of 5th level... given the power level of the world I've outlined, a true assassin is a rare thing, indeed.
Personally, I think that a death effect needs to be a death effect, not a 4d6 con effect. Isn't the tarrasque hard enough to one-hit "kill" (more like cause to enter another slumber for 1d4 years, right? why is it still alive if it rampages that often? Hasn't any character banished it, then followed it with a good-old sword on hand?)
The tarrasque isn't even on my radar. But, even if it was and if my world was built in consideration of characters that might think of taking on a tarrasque, it shouldn't be possible to one-hit "kill" it.
Right now, I think I'm going with death effects dealing 4d6 CON damage. And, I need to make the assassin a core class, like it was in AD&D. Or, design Prestige Feats that characters/npcs can take if they become assassins.
Thanks, CAFargo.