Kai, Good luck. I hope you have fun. It is often more fun to 'save the world' than just clean out an orc cave.
OTOH, I think the problem people are having (myself included) is if it's 'fun' if it is given, and the assumption that it can't be done unless it is given. My friend tells of a great epic where his high level party fought Tiamet, lost a couple of people, and yet killed the Dragon.
But how good of a story would it be if they were 5th level; it wouild be obvious that it was a 'gimme' If you were to take out a MA/old dragon, that would be amazing, yet believable. That can be earned, Wyrm will be 'given' or pure luck.
The strategy you have been toying with, means nothing for your character. Just about *any* character of *any* level can roll two 20's and get a 1 on the save. So what? It is like buying one ticket and winning the Lotto; you can't attribute it to skill.
On that topic, why not save a lot of time and effort, roll 3 20-siders now, if you get 2 20's and a 1; you win. See, it isn't skill or epic or grand story-telling. It is utilizing a technicality in the rules. Where is the challenge? You either succeed or you don't.
I love the idea, but I think it would be much better if it was a more equal opponent.
Now, for the DM.... (heh heh)
As for 'massive damage', the 50 points is based on medium, the wyrm would be at least 80 points before a check, and I would make it at least 120.
Also, this is not just some monster sitting in a dungeon. This Dragon is actively staying alive, so...
Since he displaced an entire Kingdom, he *must* know they want revenge. So is he keeping tabs? Is he scrying? Does he have spies? Is he casting divination spells? Can he buy someone off?
When they start succeeding with moving through the woods, does he do something about it? Does he ambush them? Does he sneak up through the water? Does he know they are a concerted effort, so he stays holed up? Does he use this as an opportunity to attack Kai's home island?
How hard to get/make an arrow of slaying? Is it for all dragons? Does race (color) matter? What about age?
If we know about them, so does the dragon. Does it take a special ingredient? I bet the Dragon is trying to keep darn close watch on those, or even trying to get rid of them. (whatever they/it are/is)
If the elves already have the arrows, why didn't they use them??
Once he discovers that he is being 'ambushed'; does he stay and fight, or does he leave to formulate a plan?
When he flies over the river, what is the chance he is exactly 480' away? What about 600', or 150'?
Remember, while there will be lots of archers in the area... it *may* be noticable if only one has 500' range with accuracy.
He is also a 15th lvl caster, what spells does he have? Invis? limited wish? teleport? chain lightneing? etc.
He has his horde *and* the kings horde for magic... he should have some pretty good
. Plus, how much of the elves magic items do they not have access to, because they had to leave it behind?
If the attack fails, does he hunt them down? Does he go after the island. (swim up, surprise attack... over and over)
Oh well, let us know if it works out for you...
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