Pathfinder, I'm the GM. 1½ years of playing a large portion of a bi-weekly campaign just wrapped up with the group's first true dragon showdown (a great wyrm). After defeating three liches, the group came out of a cave having spent quite a few resources but ready to face the dragon. The dragon predictably shot breath weapon into the cave before the group got out, but lingered in the cave opening in order to smack the group's front-line people around a bit. I opted for an attack and a move, opening it up for two attacks of opportunity in an arrogant manner. Well, our fighter rolled a 20, and then confirmed it with another 20. We have a house rule: 2 x 20 = possibility for a lethal blow if a THIRD roll confirms the hit once more. Well, he rolled a 19 and the dragon died. Turns out they needed the beast alive though, so his epic jubilations was quickly brought down to earth, and he had to use a Hero Point (very rarely obtainable points I give out that allow rerolls) to try and force a MISS on that final blow. He missed, and the dragon was merely knocked out instead of being outright killed.
It was a great session all in all, and holy crap can a group of 18th level characters be unstoppable! After the three liches (one a 20th level caster) encounter, our druid asked if anyone needed healing, but everyone was good to go since the cleric used "Mass Heal" during the battle (which also took out one of the lesser liches).
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Updated with more info about the circumstances: The short version is: The dragon was a silver dragon - the king of dragons, in fact - which had been possessed by an evil bastard. Dragons vanished from the players' own world a long time ago, so they are in this parallel world to save the last echoes of the great creatures, and hope to bring the last dragons back over to their own native world in order to stop an even bigger threat. Finding the last dragons, sheltered by failing magics, has been the main plot of the last 1½ years playtime, and for the groups level 15-18 efforts. Having finally found them, they had to strike a deal with an evil bastard (immortal lichlike archwizard) in order to cleanse them of the disease HE inflicted the dragons with initially, or they would die the instant they left their stasis-like chamber.
Only six dragons yet live in this altered world they are in, and although they could bring less than all six over, they wanted them all. So they struck a deal with the bad guy, and he got control of one of the dragons for the life of the others. The group wanted to undo the damage, which was why they needed the dragon downed but not dead in order to "excorcise" the evil bastard from it. They had to do so by alchemically infusing it with the blood of the evil bastard's own kin in order to break the link (since he was a bloodmage and had tainted his own blood, the "pure" bloodline of his own would effectively kill him). His kin was an ally of the party, who sacrificed herself in order to finally kill the evil bastard once the dragon was unconscious (yet another note: she had turned on the party at one point in dramatic revelation fashion, but turned back to the good guys through their valiant efforts in the fight with the liches - for once seeing reason winning out over wanton desires of vengeance).
So they saved the dragon and killed the evil bastard, and are now inbound to their own world with all six dragons as allies for the final part of the campaign. They brought the woman who sacrificed herself with them too, planning to ressurrect her if possible.
I hope it doesn't read completely like a mess. I omitted a lot of detail, but the story is actually quite engrossing (or so my players tell me).
I am closing shop on the campaign soon, but it's also been going since the summer of 2008.
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