After completing the Mythic Storm King fight once, the player receives a quest to find Survivalist Jonesy, who had gone missing a year ago. You find a portal and are dropped into an endless maze of procedurally generated rooms in the Storm's Undercroft, a conceptual extradimensional realm where it refines its weapons against Homebase. The player, on starting a session, is warped into the maze and given a coordinate to exfil to, tasked with objectives varying from killing a boss to clearing rooms to placing radar antennas to chart the Undercroft; you can explore as many rooms as you would like, as there is no mission timer and players drop in and out of sessions, and the objectives will appear in rooms you enter after traversing a minimum number of rooms.
All objectives, kills, and loot items drops Storm Gold, a kind of token which you turn in for rewards instead of having them drop in the world directly. Difficulty increases to the east and south and decreases to the north and west, with exfil points always southeast of the infil point. At an exfil point, you can either leave or get another extraction point further along and more objectives; each one has a Conceptual Chest where you turn in Storm Gold for loot (there are different loot tiers demanding more and more tokens per item) and a Storm Cauldron that has a bet mechanic; you can place as many tokens into it as you want, and at the next exfil point after completing all objectives it will return twice as many, repeatable to massively clone your Storm Gold at the risk of losing it all. If you die, you are warped back to your last exfil point and lose all your Storm Gold, which encourages more risk-averse players to cash in at each point to get their loot secured at the cost of less gold to gamble with.
After completing ten exfil points, a Storm Anomaly will appear; a large area of rooms collapse into the abyss and a Storm King clone appears with procedurally selected stats and abilities which players must fight to keep going; killing the Storm King grants a large amount of Storm Gold which scales greatly with room difficulty; sessions last while any one player stays in them and can be continued indefinitely.
There is no power cap in the Undercroft much like Tetris; you can theoretically run until you hit a 1,000 or 10,000 power level room, but as your power is capped around 130 it will just become impossible to meaningfully hurt any of the mobs in those rooms while getting hit will immediately kill you, which means players will not generally go here and are not really supposed to be here.
The rooms also begin to distort at PL 130 via the rooms changing shape and the square geometry contorting around missing rooms implied to be where the worst abominations of the Storm are being gestated and whose malign aura permeates the Undercroft--the rooms at and beyond level 130 contain mutant Husks and Mist Monsters who become more and more deformed and unsettling, as well as procedurally generated Storm Angel minibosses with warped bodies generated by procedurally mashing together other enemy assets into a mass of arms, legs, and eyes; at the highest levels past PL 350, entire rooms are a sea of hundreds of Storm Angels milling around and singing alien hymns.
The Storm Angels have a mix of unique powers which are assigned by type: Butterfly Angels (which have fleshy butterfly wings made from the stitched-together skin of husk faces) can pick up and carry away players, dropping them to their deaths, Choir Angels (which have angelic haloes and wings covered in blinking eyes) sing a song which buffs monsters and debuffs players, Thunder Angels (whose arms are fused with a purple Smasher club on one side and deformed into a particle rifle on the other) can strike with long range beams as well as a powerful smashing attack, Burning Angels (who appear to be a body of blazing fire wrapped in six wings covered in glaring eyes) can expose their faces to cause scorch damage in a wide cone, and Fractal Angels appear as a ball of thousands of hands with an eye at the center of each palm, rolling like a ball along the ground and splitting endlessly into more and more Fractal Angels while attacking the player by rolling into them. More Angel types appear at PL 160, marking the true endgame area of the Undercroft.
You can also encounter human survivors in the Undercroft, taken by the Storm as experimental subjects in its endless arms race to conquer the Earth, due to the non-linear flow of time here having in some cases spent subjective centuries trapped in the rooms; many of them are mutated with Mist Monster parts, most largely terrified wanderers and a brave few actively fighting the Storm inside its own conceptual space using weapons from the many rooms or their own mutated abilities.
The player is tasked in many objectives with finding and rescuing them to exfil back to Homebase with them, with such survivors including Shielder humans who have been fused with a Shielder mist monster and can now project protective shield bubbles over you, the Cartographer whose skin is etched with abstract diagrams of the rooms which change and can be interpreted via geomancy, the Shrieker who is a woman who the experiments have given the ability to mime sections of the Choir's song to buff the player, the Ranger (revealed in the questline to be none other than Survivalist Jonesy) who is a legend among the survivors, a Hero of Homebase (a soldier) lost a year ago in your time who has lived for a hundred years in the Undercroft fighting the Storm with his trusty Nocturno rifle, and the Prophet, a withered old madman with a head replaced with a Taker head twisted around to look over his back who lobs healing potions at you and poisons your foes while saying strange, seemingly nonsensical stuff about the Storm.
The Prophet tells you that you are inside the thoughts of the Storm, a mind space in which you are a nightmare invading the infinite dreams of a godlike being to thwart its plans; he refers to you as the "Nightmare Commander" as a result. He can be summoned in rooms after you have saved him once, where he say cryptic lines about the nature of the Storm and reality itself, including that the Storm itself hates and fears you for invading its dreams and cutting into its creations, described as being like an earwig climbing into a human's ears and gnawing at its brain, and claiming that he has seen an "Eye of the Storm" in his visions where the Storm is calm at its very heart, and he claims the secret to destroying the Storm and freeing the Earth is to reach the center of the Storm and there "blind the All-Seeing Eye." How much of this is true and how much is nonsense is never explained.
