The trip to the comet doesn't have a whole lot of player interaction. You can describe it in general terms -- high-G liftoff, weightlessness, the gold-colored core of the Celestial Sword looming ever closer, and the crashing impact penetrating into the comet's interior. PCs can now leave the capsule and enter the comet itself.
The outside of the comet is covered in a yellowish dust, blown out into a tail by the "sun-wind." Inside, where the party now finds itself, the structure consists of a red-gray material that has characteristics of both rock and flesh. Gravity is relative to the surface, ie if you try to climb the wall, you will succeed because that wall will become the floor from your perspective. Simply walking up the wall will work as will walking across the ceiling, although none of this is immediately obvious.
Tunnels and rooms within the comet grow open or closed in an organic, peristaltic manner as needed by the hive that lives there, although from a human perspective it might as well be random. This usually takes some time to accomplish; the last few rooms and hallways explored will generally remain for several turns, but if characters backtrack very far, they may see a room pinching shut or a new one slowly yawning open. Otherwise, use your favorite random dungeon generator to determine layout.
The interior is structured in several layers, each of which can consist of several "dungeon levels." The surface layer is where the party will begin, where their rocket has pierced the exterior. The surface layer consists mostly of greenhouse rooms with skylights and strange plant like things. These greenhouses are used to produce oxygen, food, and fuel. Small ventilation tunnels run into and out of the greenhouses, lined with cilia which keep the air moving. Rooms situated just under/inside the greenhouses are storage rooms where bladder caste sky-fiends (see below) wait for the technician caste to lead them to the greenhouses to water plants, collect food, or process plant waste into fuel. Other rooms on the surface include landing pod bays; the landing pods are kept unfueled until needed. Bladders wait in fuel bays nearby.
The interior layer consists of hive-like barracks rooms where the technicians and soldiers rest, as well as hatcheries and nurseries where eggs and young are cared for until they are large enough to assume the duties of their caste. Some barracks in the lowest levels of the interior have cryogenic stasis pods for keeping enough technicians alive through the long, cold period of the comet's orbit so that they can get the colony moving again once the comet begins to warm.
The deepest layer is the core. Here can be found the nuclear reactor that keeps the stasis pods running and the core areas warm enough to survive during the long winter, the queen's nest, the royal nursery (where eggs and young destined to replace the current queen are nurtured), and various stores of food and water for keeping the queen and a small retinue alive through the cold years.
The obvious hazards here are the sky fiends themselves. They look something like a cross between a flea and a hermit crab, generally ranging from the size of a dog to that of a horse. There are four main castes: bladders, technicians, soldiers, and the queen.
The bladders are the smallest and least intelligent caste, and totally harmless. They can be easily herded around, and will stop and wait where they are left unless another creature starts herding them elsewhere. They can absorb water, liquid food, or fuel and store it in their abdomens, swelling like a tick until they reach their capacity of 10 gallons. Killing a swollen bladder will cause its contents to spill all over the surrounding area; a technician can cause it to release its contents without killing it.
DCC stats: Init +0; Atk 0 (0); AC 10; HD 1d4; MV 30'; Act 1d20; SV Fort +0, Ref +0, Will +0; AL N
RC stats: AC 9; HD 1/2; MV 30'; #AT 0; D 0; Save NM; ML 7; AL N
Technicians are man-sized creatures. In some ways they are quite intelligent, but in other ways seem almost mindless. In those things they are good at (herding bladders, repairing structural damage to the comet or the landing pods, tending to the greenhouses, nurturing eggs or young) they act skillfully and decisively to overcome obstacles. In other tasks (communicating with other species, combat, disposing of non-biodegradable debris like metal weapons or armor), they kind of panic, sometimes hesitantly beginning a task, then pausing, then beginning again, and at other times ignoring the situation in favor of a task they are familiar with. A technician will prefer to leave a combat situation in order to summon soldiers, but if cornered will defend itself (albeit in a relatively ineffective panic).
DCC stats: Init +0; Atk +0 punch/slap (1d3); AC 12; HD 1d8; MV 30'; Act 1d20; SV Fort +1, Ref +1, Will +0; AL N
RC stats: AC 7; HD 1; MV 30'; #AT 1; D 1-3; Save F1; ML 7; AL N
Soldiers are the real danger to adventurers. Ranging from pony-sized up to the size of a large warhorse, where technicians have manipulative "arms" soldiers have sharp, stabbing limbs that deploy with blinding speed. They are not subtle fighters, rushing into melee as quickly as possible, although they will use cover and concealment to approach if they are taking casualties in the charge.
DCC stats: Init +2; Atk 2 stabs (1d8); AC 16; HD 2d8; MV 40'; Act 2d20; SV Fort +1, Ref +2, Will +0; AL N
RC stats: AC 3; HD 2; MV 40'; #AT 2; D 1-8; Save F2; ML 10; AL N
The queen of the colony is humongous, with a main body of about horse-size, but an abdomen devoted to egg-laying which is the size of a barn. She isn't particularly intelligent or physically capable; in a fight, she relies on her soldiers to defend her. If the queen is killed, technicians will immediately begin nurturing one of the royal young into a new queen, taking 1d4 weeks to complete the task.
DCC stats: Init +0; Atk 1 punch/slap (1d4); AC 12; HD 7d8; MV 10'; Act 1d20; SV Fort +4, Ref +3, Will +2; AL N
RC stats: AC 7; HD 7; MV 10'; #AT 1; D 1-4; Save F7; ML 9; AL N
The sky fiends are not the only hazards found on the Celestial Sword. Varrtir has been launching other adventuring parties up there to try to end this conflict, and they are not all doing very well. The alien environment and constant fighting have taken their toll, and many have gone around the bend. Some will wildly attack anything they see, others might seem normal but go berserk in combat attacking friend and foe alike. Still others might just seem friendly until they are able to steal from the PC party (with or without slaying them first), due to the last major hazard:
Malnutrition. The sky fiends as well as the food they subsist on taste horrible, but can be choked down by hungry earthlings. Even so, there is something missing from the alien biological cycle, and humans and demi-humans will slowly waste away from the deficiency. It will take several days of subsisting on food scavenged from the comet before symptoms arise. Humans can live for about three weeks with no food, so this malnutrition should take at least twice that to kill someone, although die roll penalties after a week or so aren't out of line. Particularly morbid DMs may have some of the earthlings (PC or NPC) realize that cannibalism would provide the complete nutrition that the comet's ecosystem cannot.
In order to complete the mission here, the characters must do enough damage to the sky fiends' ability to wage war that they can't invade the planet and swarm the Mushroom Tower from the ground. Random bands of adventurers could force their way into the Tower, so a good-sized horde of soldier castes could do it much more easily. A partial victory here will prevent the invasion from occurring on this orbit, kicking the can down the road for another 500 years. A complete victory will deal a final blow to the colony.
I'll leave details of just how much destruction constitutes a victory to the individual DM; here are some ideas to riff off of.
Killing sky fiends: obviously killing off the aliens themselves will do the job, but since they have the numbers for an invasion force, it's not likely the PCs will have the ability to finish them off in this way.
Destroying eggs: a lot easier than killing adults, but the queen is able to lay so many eggs, it won't matter much. Soldiers will still be able to land on Earth to attack the Tower. The best the characters can hope for here is a partial victory, with the colony using the next 500 years to rebuild their numbers to try again.
Destroying fuel: landing pods turn into suicide pods when they don't have fuel for landing. As a bonus, leading fuel-laden bladders around to different areas within the Celestial Sword allows you to kill two birds with one stone: burn the fuel and some other infrastructure at the same time. Simply destroying the fuel supply is only a partial victory.
Destroying food: whether destroying the greenhouses where the food supply grows or destroying the bladders storing the liquid food for later, reducing the food supply will affect the ability of the colony to survive the cold portion of their cometary orbit. If every bit of food is destroyed, the survivors will have to resort to cannibalism to make it through the winter, and it may be 1000 years before they pose a real invasion threat again; this is still just a partial victory.
Destroying water: killing all the water bladders will pose an annoying problem, and may be the fastest way to kill large numbers of sky fiends, but the colony has ways to recycle water used for farming or excreted as waste already, so it will eventually be recovered and stored in newly hatched bladders. Permanently destroying it (using Disintegration or other magic, or just loading bladders into landing pods and shooting them off into the void) will result in the death of the hive.
Queen: killing the queen certainly puts a damper on the sky fiends' ability to replace casualties. By itself, however, it is only a partial victory. Another queen can be nurtured from an existing egg or larva (whether this is due to some type of royal jelly or radiations from the core of the comet is left to the individual DM). Destroying the queen and the eggs is a better victory, although it doesn't necessarily prevent a suicidal vengeance invasion by the survivors. It will limit the damage of such an invasion though.
Landing pods: shooting the landing pods off into space effectively prevents the invasion this time around, making it a partial victory. These pods can be replaced by the time the comet orbits around the sun again. Also, unless the players retain one for their own use, they will have a hard time getting home.
Core heater: destroying the core heater will ensure the death of the colony. The cryogenic stasis pods are not automatic, so there must be attendants active outside the pods to awaken those in stasis. Without the core heater, the queen and her attendants will all freeze during the cold part of the orbit. If the core heater is destroyed early enough in the adventure, the sky fiends will attempt to send landing pods out with technicians, eggs, and enough bladders to get new nests started. This would normally be a suicide mission, but with nothing left to lose, it's worth trying. If the players have already destroyed the landers, technicians, eggs, or bladders, this whole plan falls apart, earning the party a full victory.
Once the party has dealt a fatal blow to the hive, inflicted all the damage they think they can do and still make it back home, or simply get beaten so badly they have to retreat or die, it's time to consider how to get them back to the campaign world (or move them on to another alien planet if that's how you roll). From the time the Celestial Sword is first sighted in the sky to the time it is no longer feasible to get back should be a total of about 18 months, so it is possible to fit in a long journey to the Mushroom Tower and a lengthy guerrilla war on the Sword itself.
Some possible means to return to the home planet include repairing the rocket the party came in on (or one that brought an NPC party), hijacking a sky fiend landing pod, or calling for help from the Mushroom Tower. Failing all of these, the party can attempt to use cryosleep pods to wait out the long winter, returning in 500 years to a radically altered campaign world.
Repairing a rocket from the Tower will not be easy, given the unfamiliarity the typical sword-and-sorcery character has with space age technology. However, given time and enough wrecked rockets to pick over, as well as a generous DM, an intelligent character may be able to cobble together a working vehicle. They will definitely need to steal some fuel from the sky fiends; hope they didn't kill all the fuel bladders already.
Stealing a landing pod is a little quicker and easier, but humans or demi-humans have no hope of controlling it without help. Besides fueling it, they will also need to snatch up a technician and get its knowledge somehow. A charmed or otherwise mind-controlled technician will cooperate, and an ESP spell can allow a wizard to glean the necessary information to control the machine without the technician's cooperation.
Calling for help won't be easy. Most magic spells don't carry to interplanetary distances. Perhaps a character can figure out how to repair and use the communications gear from a rocket wreck. If a character messed around with a psychic radio in the Mushroom Tower, maybe there is a lingering connection that allows them to contact Varrtir. He won't send a rescue team just to evacuate a party that's in over its head, though. If they haven't ended the threat of the sky fiends once and for all, they'd better get cracking.
At any rate, comms will help with any extraction technique the party chooses to pursue. They can get a walkthrough repairing a rocket, Varrtir can send a rescue rocket to pull them out (this one won't have to crash into the comet if the sky fiends aren't manning defenses), or at the very least Varrtir won't use point defense weaponry against a landing pod if he knows it's not an invader.
I suppose many parties won't be satisfied with saving the world. For those mercenary sorts, they can earn some material gain by bringing back pieces of sky fiends and/or samples of their plants from the greenhouses. Earthly wizards and alchemists would be interested in buying these for research. The connection with the sky in general and a comet in particular may make them useful for flight, weather, or divination related spells or magic items. If that's not quite good enough, the DM can sprinkle some gems around the comet, possibly embedded in the walls or used as part of the technology in the landing pods, greenhouses, or core heater.
Campaign Continuity Bonus: If the sky fiends were able to launch landing pods with either soldiers or the basis of new nests, the sky fiends may return in some capacity in later adventures.