Designing Demiplanes (GEP)

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Because demiplanes are such unique areas, DMs should feel free to let their creative impulses run wild. A realm of solid bone, animated weapons and armor, or intelligent bugs-it's all possible. These pocket dimensions are places where literally anything can happen, and an adventure (or even a campaign) set within the confines of a demiplane promises many challenges and memorable moments.
The first rule a DM should know when designing demiplanes is that there are no rules. Turning the multiverse upside down is part of the charm of designing a pocket dimension. That being said, Dungeon Masters should make sure their demiplanes have a measure of internal consistency-unless, of course, the area's prime characteristic is its randomness. Thus, a demiplane composed of superheated water should probably not have too many creatures composed of ice inhabiting its expanse, unless the DM can come up with compelling reason for their presence. The key to designing demiplanes is to make them interesting. Demiplanes that mimic the other planes exactly probably won't excite players' imaginations
==Generating Random Demiplanes==
Of course, some planewalkers simply poke around every area of the color curtain that they can, and a DM might not have the opportunity (or the time) to design a hundred unique demiplanes. In such situations, DMs can consult the following tables, designed especially for the random creation of demiplanes. Although the tables don't heavily detail individual demiplanes, they do provide idea seeds that DMs can easily transform into entire adventures.
First, the DM should determine the color of the demiplane's specific curtain area. He should simply roll a d20 and find the results on Table VII: Curtain Color below. If a DM wants total randomness, he should roll the d4 to determine how many times he should roll on the following tables. For example, the DM rolls a 2 on a d4. He then rolls twice on the table and determines that the curtain is a swirling combination of rose and sable hues.
Second, the DM should roll on Table VIII: Demiplane Characteristics. By rolling more than once, strange combinations come together that sometimes prove more interesting than a single characteristic alone (reroll duplicate results). For example, the DM rolls a d20 twice, and gets a 20 and a 3. According to the table, this demiplane negates the use of magic and possesses a different time stream (time moves 1d4+1x slower than the rest of the multiverse).
Third, the Dungeon Master rolls on Table IX: Demiplane Inhabitants. By rolling more than once, a DM can generate unlikely groups of inhabitants. This provides the Dungeon Master with an opportunity to create unique reasons for these creatures' existence on the demiplane. If a DM can't make sense of a randomly generated result, she should discard it. For example, the DM rolls three times on Table IX, generating an 8, a 3, and a 14. This means that beholders, humans, and some free-willed golems inhabit this demiplane. The Dungeon Master then decides he doesn't like the beholders in the picture, and jettisons them. Now, he's left with a demiplane where humans and free-willed golems live together in an eclectic community. Alternatively, he may decide that the humans and golems are in the midst of a furious war.
Finally, the DM needs to decide the size of the demiplane. Sometimes the results of both the previous tables can help a DM decide upon the size of an area; otherwise he can roll on Table X: Size. Unlike the other tables, the DM should roll only once on this table, unless given different instructions. Note that all sizes occur in diameters (suggesting a circle or sphere); however, the Dungeon Master should shape the demiplane however he sees fit. The DM rolls a 7, and determines that his demiplane-in-the-making is 3 miles in diameter. Sometimes a demiplane's characteristics suggest specific types of terrain, but the DM arbitrarily decides that his demiplane is rocky, craggy, and rough, with very little vegetation.
Now, the DM must flesh out the demiplane in response to the actions and questions of his players. Depending upon his inclinations, this demiplane could act as a side trek for a larger adventure, or it could function as an integral party of his ongoing campaign.
 Table 7: Curtain Color
Roll 1d20, combine any multiple rolls as swirls	Roll 1d20, combine any multiple rolls as swirls
01 	Azure
02 	Caramel
03 	Cerise
04 	Cobalt
05 	Copper
06 	Emerald
07 	Gold
08 	Ivory
09 	Lime
10 	Olive
11 	Rose
12 	Ruby
13 	Sable
14 	Saffron
15 	Sapphire
16 	Scarlet
17 	Silver
18 	Tan
19 	Teal
20 	Verdant
 Table 8: Demiplane Characteristics
Roll d20, combine multiple rolls	Roll d20, combine multiple rolls
01 	All wizard spells cast one level higher
02 	Completely composed of (roll 1d8):
 	01 Bones
 	02 Rugs
 	03 Copper coins.
 	04 Feathers
 	05 Hair
 	06 Mucous
 	07 Slime (possibly monstrous, as per MONSTROUS MANUAL tome)
 	08 Worms
03 	Dead magic (magic doesn't work here at all)
04 	Double strength gravity
05 	Gelatinous
06 	Grassy plains
07 	Jungle
08 	Lacking a Border Ethereal
09 	Landforms melt and reform in random manner
10 	Marshy
11 	Shares reduced characteristics of a random Inner Plane
12 	Mountainous
13 	No gravity
14 	Poisonous atmosphere (to nonnatives)
15 	Possesses two Border Ethereal layers
16 	Priests cast spells as if 1 level higher
17 	Rocked by constant "earthquakes"
18 	Sentient
19 	Storm-wracked
20 	Time slower than in Deep by 1d4+1x
 Table 9: Demiplane Inhabitants
Roll	1d20, combine multiple rolls
01 	Aasimon
02 	Baatezu
03 	Beholders and beholderkin
04 	Roll on Ethereal Encounter Table (page 95)
05 	Colonized by splinter race/group of (roll 1d12):
 	01 Aasimar
 	02 Bariaur
 	03 Dwarves
 	04 Elves
 	05 Genasi (roll for type)
 	06 Gnomes
 	07 Halflings
 	08 Humans
 	09 Minotaur
 	10 Orcs and/or goblinoids
 	11 Rogue modrons
 	12 Tieflings
06 	Dinosaurs
07 	Dopplegangers
08 	Dragon(s)
09 	Empty/dead (don't combine with other results)
10 	Giants
11 	Illithids and illithidkin
12 	Intelligent fungus and myconids
13 	Mephit (if appropriate elemental mix is present)
14 	Refuge for free-willed golems
15 	Sahuagin (if water is present)
16 	Secret fortress of 10th- to 20th-level wizard
17 	Secret redoubt of multilevel wizard's cabal
18 	Tanar'ri
19 	Undead
20 	Yugoloths
 Table 10: Size*
Roll	1d12 once
01 	10 feet diameter
02 	100 feet diameter
03 	1,000 feet diameter
04 	2,000 feet diameter
05 	1 mile diameter
06 	2 miles diameter
07 	3 miles diameter
08 	6 miles diameter
09 	12 miles diameter
10 	25 miles diameter
11 	50 miles diameter
12 	50 miles + roll again and sum the results
: * Though size is given in diameter, the demiplane can take any outline desired by the DM.