* Good vehicle (massive constrained sandbox) for exploration-focused (rather than story-focused games).
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* The joy of unscripted discovery that comes with the above two features (requires effective megadungeon design).
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* "Story" that is emergent, undetermined, not guaranteed, retrospective.
I will give an example of play from Thursday night.
-The party read about a sage from the "Invisible College" who was hiring "adventures of highest caliber" for an expedition. They tried to find the college to inquire about being hired, but discovered that not only is this wizard's academy invisible, one needs to actually be invisible to enter it.
-The party then consulted a "fortune teller" (who may well be a charlatan) for advice on where to find treasure, and was informed of the existence of a "moth-man" in the north end of the buried undercity.
-At a Yawning Portal analog they were lowered by winch into the undercity. While attempting to find a path to the north, they encountered a swarm of cranium rats. These psionic rodents offered information on how the party could reach their destination, if the party would steal the "claws of Bast," to which the party agreed. While making their way to the buried temple of Bast, they encountered and a fought a group of beastmen; two beastmen were slain before the other two fled. Somewhat injured, the party holed up in what they determined to be the remnants of a paper-maker's shop, only to be attacked by a paper golem.
-After a short rest and some healing, they entered the buried temple. The gnome used his mechanical mouse and some wads of balled up paper distract the panther guardians, then snuck in and removed the wooden claws from hands of the stone statue of Bast. The party returned to their rat overlords, who fulfilled their side of the bargain with psychically bestowed directions.
-Along the path to this moth-man, the party encountered a group of kobolds. There was a quick fight, which further depleted the party's resources, but the group was quickly victorious. Now basically out of healing and getting low on spells, the party pressed on. Finally they encountered the moth-man in an underground plaza. The creature, with glowing sleep-inducing eyes and an insectile tongue that drained life-energy, was a tough fight, but a critical hit finished it. The party retrieved some treasure and equipment (mostly from bodies of other adventurers who had been slain by the moth-man). Extremely wary of any further fights, the party retraced their steps to exit the undercity. On their way out there was a random encounter, but fortunately for them only a single beastman who didn't like the odds. The party fled quickly before the beastman could return with allies.
So in the course of about 3.5 hours, the players had some NPC-based RP (talking with the fortune teller and negotiating with the cranial rats), four quick fights (I don't think more than 10-20 minutes each), a puzzle (getting past the panther guardians), as well as some other miscellaneous exploration and intra-party RP. This seems to be a good mix for us. I had no idea at all what would happen before play began. The players had a lot of fun - there was a mixture of known elements (kobolds) and the bizarre (cranium rats, the mothman) to keep them guessing, and heading toward the end there was definite tension on whether the party would make it.