Layered Planes, Uncanny Sense, and Uncanny Auras

"Reality is like an egg, Everddyne. No, that's not quite it.... Reality is like- baklava but- ugh, no. So reality is like the air, it has multiple kinds of gases in it because we can separate them through some processes. But to the naked eye they are all in the same layer. To have a level of sight... an uncanny sense, It allows you to see the gasses in their layers. So- I guess it is like baklava. Hmmm? Yes I'd like a pastry dear, how did you know?"

— Nixsommer Northwrought on how uncanny senses can perceive other planes

Layered Planes

The Planes are widely known in the fact that they exist, though their workings are not commonly understood. The usual view is that other planes are far away and hard to access. This is correct in the sense that planes are typically hard to access without extenuating circumstances, but not all are the common definition of far away. In fact several planes can occupy the same space without much crossover; This phenomena is referred to as Layered Planes. Layered Planes are especially common among the extradimensional lairs of some beings.

Layered Planes and Enchanted Items

Layered planes and how they work are linked closely to bags of holding and other pocket dimension enchanted items. The items make use of space provided by the concept of layered planes. There is technically infinite space in almost any position if you have the power to access it, but it is easier to pull off a chunk of an already existing plane than pull empty space apart from scratch. However, Taking advantage of a pre-existing plane is not without its perils. The piece of plane which you take will be naturally attracted to the rest of its plane, similar to how a compass needle's end is attracted to true north. It is a fine balance to strike, to tear off enough to be stable, but not enough for it to snap back to the rest of its 'mother' plane.
This concept is why one should never place one pocket dimensional item in another. The effect of two items drawn from the same plane coming into such close contact draws their metaphysical pull together and snaps both back into the mother plane. This is called Adjacent Plane Crossover, and it can occur with planes the are similar enough in nature that they do not naturally repel each other. From this snapback a temporary portal is created that has a vacuous pull until energy equalizes, drawing any nearby object or creature not secured into the items' mother plane. If the items are not from the same or a similarly natured plane, it causes Opposing Plane Crossover, where there is a chance for even more catastrophic side effects.

Table of Opposing Plane Crossover Events

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Roll	effect
1	The ground shudders as a hole to both item's connected plane is shredded open. The holes react violently sucking everything in a 100ft(30.5m) radius. Then after 3 rounds it explodes causing 20d10 force damage to everything in a 50ft(15.2m) radius and 1/2 that damage rounded up for the next additional 50ft(15.2m). In the aftermath is left one random aberration of at least CR 3. One rank of Uncanny Sense may be given to PCs in the blast radius at the GM's discretion.
2-10	Two random aberrations of CR3 or higher appear and attack.
11-20	One of the items overpowers the other and creates a temporary portal to it's mother plane. The portal sucks in everything within a 20ft(6.1m) radius. A creature may escape this pull with a successful CR15 DEX or STR check, otherwise they are drawn into the portal and appear in the plane it is connected to 25ft(7.6m) above the ground.
21-30	The planes rip apart and slam back together creating a shockwave causing 3d10 thunder damage in the 30ft(9.1m) radius. Additionally, a boom is heard loudly in a 5 mile(1.6km) radius and faintly for another 15 miles(24.1km).
31-40	All plant life immediately wilts and rots and dark gas disperses over a 50ft(15.2m) radius. All creatures in the radius take 4d10 acid damage halved on a DC20 CON saving throw and are poisoned for 1d4 days as their molecular structure is damaged by the planes temporarily shredding.
41-50	As the planes shudder a tiny crack opens up to an unexpected place, a strange scent becomes heavy in the air. All creatures make a DC17 WILL saving throw, at a failure they take 2d6 psychic damage and fall unconscious for 1d4 hours, on a success they fall unconscious for 1d4 hours.
51-60	All creatures above 5 intelligence must make a DC17 WIZ saving throw or gain a moderate short term madness.
61-70	Sharp ringing fills the area. All creatures must make a DC17 CON saving throw or become horribly dizzy and have disadvantage on all DEX checks and saving throws for 2d4hrs.
71-80	A piecing drone shatters the atmosphere, all creatures in a 1 mile(1.6km) with under 4 intelligence are driven mad for 1d8 hours or until a successful DC17 WIZ (animal handling) check.
81-90	As the items are destroyed they explode. The blast has a radius of 50ft(15.2)and causes 2d6 force damage.
91-99	The fabric of reality shudders, all creatures within a 20ft(6.1m) radius are teleported to a random location 10 miles away.
100	As the items are destroyed they explode, leaving an acrid smell that cannot be washed off for 2d6 days.

Uncanny Sense

Uncanny Sense is the ability to perceive things on adjacent, layered planes to the one you are residing on. Mortals can be born with it in exceedingly rare circumstances, but typically gain it later in life as an afflicted ability. It's most basic function at the lowest ranks of the ability works like a sixth sense. A person will be more sensitive to extra planer activity on adjacent planes, beings that produce an otherworldly aura such as ghosts, Fae, and Archios. Any strong magical aura manifests in relation to a plane, so even extraordinarily powerful mortal magicians may be detected by Uncanny Sense. At higher levels Uncanny Sense has more effects and begins to come with drawbacks.

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Rank	Boon	Bane
1	Gain skill 'Aural Perception'	N/A
2	+1 Aural Perception	N/A
3	+1 Aural Perception	-1 On saving throws against being charmed or stunned by Aberrations.
4	+1 Aural Perception	Gain 1 rank in Uncanny Aura
5	+2 Aural Perception
6	+1 Aural Perception	Gain 1 Rank in UA
7	+1 Aural Perception	You can now see visually perceive some things occurring in planes layered against the one you are on. Gain 1 rank in UA
8	+1 Aural Perception	Gain 1 Rank in UA
9	+1 Aural Perception
10	+1 Aural Perception	Permanent Major Madness Gain 1 Rank in UA

Gaining Uncanny Sense

The game mechanic is only unlocked by events during the game.
A character may gain a rank in Uncanny sense through preplanned events, interacting with specific NPCs, battling certain monsters, or sacrificing health or ability score points in a ritual. If a DM chooses, a character may start with rank(s) in Uncanny Sense which may suit some campaigns well.

Alex's character the fighter Malcome has successfully convinced a powerful outerplaner being to not drag a city into his realm because "He liked the aesthetic." Due to this the being granted a boon, one rank of Uncanny Sense for his cleverness.

Meta information for Uncanny Sense

Overall ranks in Uncanny Sense are at the DMs discretion to use in enhancing the atmosphere of a game with a creepy and/or otherworldly tone. Uncanny Sense is an integral facet of Hydragyrum as a setting, not because it is a essential part of every game, but because it is optional. It's purpose is to allow games that are heavy in cosmic/space horror believably exist in the same in world time and area as a more lighthearted adventure games so you can play without retconning the setting and stay on the mortal plane for possibly the entire campaign.

Uncanny Aura

Uncanny Aura is a measurement for how much of a noticeable energy imprint a creature, item, or phenomena. This aura is what something would perceive using Uncanny Sense. The higher something Uncanny Aura is the more noticeable they are.
TABLE HERE

Special thanks to DeaadxSassy on twitch for bouncing random ideas to me for this ones big ole tables.

Opposing plane crossovers

Opposing Plane crossovers react in fluctuating but still somewhat predictable manner. While a specific effect may not be possible to predict there are trends that these anomalies follow. If the crossover is mild they will often simply repel each other before snapping back to their respective mother planes resulting in only the destruction of both items and at most knocking some paintings and knickknacks around. If the crossover is more intense the reaction can be incredibly dangerous, dragging creatures and objects through to the plane at the point of the crossover, sapping lifeforce from nearby entities, or just outright exploding.

by Drackni Waterwisper AKA dragonmagpie