In my upcoming Cold Frontier campaign, one of the player characters has the "Weirdness Magnet" disadvantage - a GURPS trait that constantly causes strange things to happen to them. And one of the first strange things he will encounter in the Cold Frontier is a Mothman.
Initially, that was all I had planned for the Mothman - a strange presence that PC encounters again and again. But after thinking it over for a while, I have come up with an idea for expanding the role of the Mothman: The Mothmen were the Prehuman race of beings which first colonized the continent of Nardhome and left their ruins behind. It was them who created the God Chamber and in the end created the God Tekel, though it devastated their civilization in the process. The Mothman encountered by the PCs is not an actual member of their race, but a psychic remnant of sorts left over from their God-Creation process which attempts to manipulate events for its own purposes. To what end? It will likely revolve around whatever climax the campaign builds up to. Whenever the PCs manage to "kill" it, it just dissolves and reforms later
This has, of course, several major implications for the campaign. First, let's look at the God Chamber and what its usage implies.
What if the emotions of every sapient being in Nardhome influence what kind of God is created when the God Chamber is activated? Perhaps the new God is a Gestalt entity which merges the consciousness of everyone on the continent into a new being, though the one at the helm of the God Chamber becomes its primary consciousness - similar to the Transcendence Victory of Sid Meier's Alpha Centauri.
The Mothmen created Tekel, the God of Fear, which implies that they were deeply afraid of something. Perhaps some other alien species which they waged a war against across the stars? Perhaps that other species was winning, and Nardhome represented a remote outpost where they attempted to create their last, best hope, a being powerful enough to take the fight back to their enemies. As the Black Pyramid housing the God Chamber neared completion, the outpost was discovered by their enemies and attacked. They activated it and the consciousness of every Mothmen on the continent merged - but that also included many of their enemies. Tekel was able to end the war, in a way, but both civilizations fell, never to rise again.
This means that most Mothman ruins on Nardhome will have served either military purposes or supporting the God Chamber project in one way or another. The Mothman origin should not be immediately apparent, but their construction, furniture, and artifacts should all be disturbingly nonhuman. Furthermore, there may be psychic remnants of whatever alien species the Mothmen fought - as well as skeletons from both species.
The next civilization to settle Nardhome was the First City. They were, well, basically fantasy Transhumanists. They used their magics to reshape themselves, their children, and their servants - the former being turned into elves, the others into most of the other nonhuman races common to D&D worlds. And they not only reshaped humans, but also animals, plants, and... diseases. When the God Chamber project was discovered, studied, and finally in the process of being reactivated, the Nardhome colony declared independence from the First City itself, as many back home thought activating it would be a bad idea, while others wanted to have more control over the project itself. The whole situation degenerated into biological warfare which caused widespread suffering both in Nardhome and back home, and appropriately the resulting God was Nyros, the God of Evolution and Disease - and Transcendence would certainly fall into its portfolio. (And note that Nyros would generally be in favor of reactivating the God Chamber - and there will likely be a number of his priests popping up with all sorts of interesting apocrypha of his faith which give further hints to the existence and nature of the God Chamber. Getting cooperation from a faith which is generally seen as at least selfish if not outright evil should be interesting for the PCs...)
Finally, there were the Old Native settlers. One of their sorcerers figured out the nature of the God Chamber, seized control of the region surrounding the central crater with his minions (many of them undead, others just abominable in general), while the other Natives fought his forces in a devastating war. Hunger was widespread under these conditions. When the God Chamber was activated, it turned out that the sorcerer had miscalculated - the transformation was incomplete after absorbing everyone's life force (the "Great Darkness" of Coastal Tribe legend), and most of its essence remained trapped inside the Pyramid as an Immortal Ichor without transcending, waiting for someone to compel into either completing the God Chamber properly or release all the souls trapped in there (it is suffering from a very bad case of split personality...). The rest dissipated across Nardhome and became entities of hunger and death, like the Wendigo.
The products of the God Chamber - two fairly malevolent deities as well as numerous less potent malignant entities - might convince the PCs that activating the God Chamber will always produce bad results. But if they do a lot of research, they might realize that this is not actually the case. All the sapient beings on Nardhome will join into a new God, and their overall disposition will determine the nature of the God. If there is a lot of strife on the continent, the God will be aggressive. But if the PCs created a new society full of harmony and other positive emotions, then the new God will reflect that as well. Of course, if they want to be ethical about this, they should make sure that only those who want to become part of a God remain on the continent - but as soon as they announce their intentions, numerous interested groups will attempt to seize the continent for themselves, which means that there will be lot of all that strife and all those negative emotions the PCs wanted to avoid. But hey, nobody said that becoming a God was easy...
Back to the Mothman the PCs will encounter. Since the PCs will work their way towards the God Chamber throughout the campaign, it presumably wants the God Chamber to be used and thus manipulates fate to ensure that this happens - which may also mean manipulating their competition into showing up on the scene.
But beyond that, it can also serve a more "meta-gaming" use in the campaign. One problem that most RPG groups have to face is what to do with player characters whose players are absent - especially if there is no plausible reasons for those PCs to be absent, nor for why they should reappear all of a sudden when the player reappears. One member of my current gaming groups came up with some good ideas for how to solve that problem. In a fantasy campaign such characters became "mana-apathic", a magical disease that struck from time to time which removed any kind of personal drive for a time and made them susceptible to orders from others. In a current fantasy/science fiction crossover campain, the PCs in question are swallowed by a particularly disgusting demon and then are later spat out near people they are familiar with - in other words, the other PCs (the PCs are not the only ones to be affected this way, of course - in fact, the "science fiction worlds" are currently in quite a lot of panic because of this demon).
For the Cold Frontier campaign, I will simply have the Mothman stun the relevant character (and remember, they can manipulate fate so that they can catch the character alone), abduct him, wipe his memories and then return him later to whereever the PCs are when the player returns. The players will hopefully understand the "meta-reason" and go along with this, but the PCs will only find some strange footprints at best, though they will hear stories about "Mothman Abductions" from native tribes. If they use magic to get their memories restored, they will remember the Mothman stunning them. If they do so repeatedly after multiple abductions, they will eventually remember strange, nightmarish scenes of the Mothman examining them and experimenting on them (possibly implanting some strange, small stones into their bodies?), similar to the stories of UFO abductions.
This will firmly establish the Mothman as a weird "background annoyance/menance" which they cannot do anything about right now without actually being resentful over it (since the PC is always returned unharmed to wherever they are needed. Thus, the shock when they discover the truth about the Prehuman ruins should be particularly effective, since they have seen their creators from the very start in the campaign.