Mi-go are creatures of contradictions: Plant type creatures that look and fly like arthropods, fungi vastly smarter than most men, surgeons and scientists given to galactic conquest, technologists whose devices can mirror most magical effects, creatures without organs who collect brains. They worship dark Outer Gods (Shub-Niggurath especially) from the void of space and war with elder things and other creatures from beyond the stars.
In a campaign that already features otherworldly or Lovecraftian threats, mi-go simply raise the stakes, as they have numbers and technology on their side. Heck, they even like hill-country caves—you could stick them in the Caves of Chaos without raising a sweat.
Then again, the introduction of mi-go could make your campaign take a hard left turn. Imagine this: Your party’s paladin is charged with retrieving the body of a deceased prelate for interment in the Holy City. The trip there is the usual—a few lizardfolk, a troublesome hag, some peat bogs. But when they arrive they find the prelate autopsied, his brain removed and sitting in a strange humming jar, and insect-like fungi buzzing in an argument about which moon to deliver the cargo to. Suddenly your campaign just got weird.
Speaking of weird, I think the mi-go’s fungal nature is also a great way of uniting the old weird of Lovecraft with the New Weird of folks like Jeff VanderMeer. If you fell in love with Ambergris but want an even darker setting for your campaign, have mi-go ally with myceloids in one of your fantasy cities and let the harvest begin.
Tainted, possibly fiendish satyr servants of Shub-Niggurath have started capturing women of every species, apparently determined to breed their way back to the Time Before Time. Adventurers who try to confront these foul beasts find that they are increasingly well armed, smearing glowing alchemical goo over themselves to bolster their strength and using pitchforks that spit lightning and worse. The satyrs are being supplied by mi-go, who worship the same dark deity. Interestingly, some of the same stone circles that the satyrs use to slip into Faerie also open to strange pulsing caverns below stranger skies.
Clockwork creatures aren’t the next step forward in construct design. They are in fact cunning traps waiting to be sprung. Their designs were carefully seeded throughout the world by mi-go, who need only hum a few bars in their strange language to direct the clockwork machines to do their bidding.
The Corpse Train is a hurtling monument to death—a lightning rail car that spirals across the tidally locked planet of Duwalin, shuttling corpses from Gloomside to the sunbaked desert of the Kiln. This gruesome transport is necessary to keep the necromantic energy of the Glooms from raising the corpses as zombies or other undead. Adventurers are often called upon to keep the giant train free from ghouls, ghasts, and worse, as every stop, no matter how brief, invites the living dead. But they face an entirely new threat when mi-go take over three whole cars and begin harvesting brains, testing weapons, and performing breeding and grafting experiments even as the train rockets ever onward.
—Pathfinder Adventure Path #46 86–87 & Pathfinder Bestiary 4 193
Obviously, both the Carrion Crown and Iron Gods Adventure Paths have a lot to say about mi-go and their technology, especially the full description in Pathfinder Adventure Path #46: Wake of the Watcher as well as PAP #88: Valley of the Brain Collectors.
I’m finally doing something about my lack of firsthand Lovecraft knowledge! Or I will soon, at least—my brother got me the gorgeous new Klinger annotated collection for my birthday (along with a copy of Eldritch Horror and Jean-Christophe Valtat’s Luminous Chaos).