In Operation Unfathomable, 1st-level characters, the most fragile of adventurers, get to explore and loot a partially pacified area of the vast and mythic Underworld that would, under most circumstances, be way beyond their ability to survive. The characters, creatures, and deities occupy unusual physical spaces where you can stage set-piece battles or provide hiding places for cowering PCs. Beyond the initial hook, there is no plot except what emerges from play. Just by entering the Underworld, the player characters may run afoul of factions that will hound them to the ends of the earth, or they might establish allegiances capable of saving them down the road.
In combat, many of the NPCs and creatures (to say nothing of the Chaos Godlings) would very quickly mop the floor with a gaggle of 1st-level PCs. Some things are simply not meant to be fought, so you might want to let the players know this up front. Likewise, many of the treasures up for grabs in the adventure are very powerful magic items that might seem inappropriatae for low-level characters. But this is the Underworld!
You could use Operation Unfathomable and its companion volume, Odious Uplands, to begin a campaign of your own, centering around exploration of the Underworld. Fort Enterprise, the closest thing to civilization in Stonespear Province, provides a possible long-term base of operations and multiple avenues for further adventures. You could also use the book as an idea mine and plunder its contents ruthlessly for NPCs, monsters, treasures, situations, and scenarios. Or you could read this book, get all fired up about your own ideas, put it up for sale on eBay, and then proceed to write the next great RPG setting.
Learn more at the October 2016 Kickstarter campaign page. - Bryce Lynch review at TenFootPole.org - August 2018 Sorcerer's Skull interview with designer Jason Sholtis.
The Operation Unfathomable Player's Guide (36pp, retail price $4) duplicates the spells and items from the Operation Unfathomable corebook.
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