Invisible Fortress:Reality Engine
Created in the First Age, Reality Engines are magnificent artifacts which allow sculpting reality from chaos (or turning reality into chaos), and pushing back the Feywild. In the First Age, they were used to expand the size of Creation, and to modify areas within Creation. Because of the effect they have on fey creatures, they have also occasionally been used to defend areas from them.
A reality engineer can use a Reality Engine to turn an area of a Feywild pocket with a radius of up to 2 miles around the engine into normal Creation. Doing so requires succeeding on an Intelligence (Nature
Intelligence (Nature)skillYour Intelligence (Nature) check measures your ability to recall lore about terrain, plants and animals, the weather, and natural cycles.) skill check, a Charisma (Performance
Charisma (Performance)skillYour Charisma (Performance) check determines how well you can delight an audience with music, dance, acting, storytelling, or some other form of entertainment.) skill check, and an Intelligence (Reality Engine) tool check, all at DC 30. The stable area created must be something that could function according to the normal rules of nature. All three checks must be made, you cannot stop in the middle. This change lasts indefinitely, even if the engine is turned off or moved, although an active engine will prevent new Feywild pockets from forming in the area.
The Reality Engine can also be used to reshape an area of Creation as the engineer pleases. While the normal rules of nature will apply again as soon as the engine is turned off or moved out of the area, the reshaping can be anything the engineer can imagine. Doing so requires the same three checks, but with a DC 35, and a character without proficiency in Reality Engines makes all three checks with disadvantage. If all three checks fail, roll a d20. On a 1-9, the area targeted becomes a Feywild pocket. Fey creatures cannot make use of this feature.
Finally, an area of Creation can be intentionally transformed into a Feywild pocket (or if the area borders the Feywild, it can be merged with the Feywild). This requires the same set of checks, at DC 40, and a character without proficiency in Reality Engines makes all three checks with disadvantage.
Each check that fails while operating a Reality Engine deals radiant damage to the engineer equal to (the amount the check failed by)d4. If the engineer is immune to radiant damage, this damage is dealt as though they had resistance to radiant damage. If the engineer has resistance to radiant damage, this damage is dealt as though they had no resistance. If the engineer is not immune or resistant to radiant damage, this damage is dealt as though they had vulnerability to radiant damage.
If a fey creature uses a Reality Engine, they subtract d8 from each check. If the engine would deal damage to them, the damage dice are d6 instead of d4.
A number of Reality Engines have been lost in the Feywild thanks to the Balorian Crusade. The engines were placed near the edges of Creation, and when the Feywild encroached on Creation due to the crusade, the engines and their areas of influence were engulfed. Any of them that were active at the time are presumably generating a "Creation pocket" within the Feywild.