[Layer]It's got a good beat, but you can't dance to it.

[Elvaris]

The two dretches sat in a rare quiet spot on the layer of infinite portals, regarding the one portal that had been the talk of the Abyss for some time.

"*So, any word on when they'll try again?" asked one.
"Rumor has it they're not going in again. Too many losses for one lousy layer," came the reply.
"Funny, it wasn't one lousy layer when they were organizing the first two assaults."
"Are you volunteering for a third?"
"What? Me? By the Styx, no."
"Then keep your voice down about the first two. A complaint in the wrong ear could cost you your head."
"Still, how did she do it? I mean, how does a lone human defeat an entire army of demons?"
"Two armies, and nobody knows exactly what she is. But they say she had access to a type of magic that made her extremely powerful."
"So, what... Elemental magic? Some weird otherworldly stuff?"
"Some kind of musical magic, I think."
"What? She's a BARD??!"
"Keep it down or you're going to get us both killed."
"A bard? The unstoppable hordes got decimated by a bard?"
"Twice. Don't mock what you don't understand. Not a single demon made it back out of that portal. Not one. That layer's bad news, and we should talk about something else... somewhere else."

* Translated from the Abyssal.

Adagio, 1,679,457^th layer of the Abyss

• Normal Gravity
• Normal Time
• Magically Morphic - Epic-level Seekers of the Song can alter this level as deities could a divinely morphic plane. All others (including deities) treat this layer as alterable.
• Mildly evil-aligned
• Enhanced magic - All effects generated by the seeker music ability are maximized (As if the Maximize Spell-Like Ability feat had been used on them).
• Maddening Song: At the conclusion of each day anyone not native to this layer is exposed to this infectious ditty, they must succeed on a Will saving throw (DC 12 + number of days exposed) or suffer one point of Wisdom damage. Should a being's Wisdom reach 0 in this manner, they gain the tuneless template.

Prior to the arrival of Whistler, Adagio was just another nameless, nondescript layer amidst the infinity that is the Abyss. Since she infused the layer with the primal music of the Seekers, it has become something extraordinary: a pocket of the Abyss defined entirely by music. Every inhabitant, every pool of fire, every tortured soul has been touched in some way by this song of power giving those who can harness the power of the song the ability to shape things as they see fit.

[Elvaris]

"So she just sang at demon lords and they died?"
"Whistled, apparently."
"Oh, well... that makes all the difference, then. If she whistled..."
"Look, nobody knows what happened. Maybe they died, maybe they're trapped, maybe... I don't know, but I'm not going in to find out."
"Okay, okay. What's it to you anyway? And how do you know so much about it if nobody made it out?"
"I saw her. I was here when she went into the portal. She looked right at me, and she whistled at me... This hollow ringing whistle... and I couldn't move. Couldn't do anything. Just watched as she strolled through the portal as if she owned the place. I'm telling you, she wasn't human. Wasn't anything I recognized. She was just..."

The conversation drifted to a stop as the dretch got lost in his recollections.

"I guess she did."
"What?"
"Own the place. You said she walked in as if she owned the place."
"Oh. Yeah. Well, she does now. And she can have it for all I care."

Whistler

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Female Aetherkin Bard 1 Seeker of the Song 15
Medium Outsider
Hit Dice: 11d8 + 16d6 + 54 (182 hp)
Initiative: +4 (+2 Dex, +2 )
Speed: 40
Armor Class: 25 (+2 Dex, +8 Bracers of Armor, +5 Ring of Protection), touch 25, flatfooted 23
Base Attack: +21/+16/+11/+6 Grapple: +23 (+4 Epic)
Attack: +27 melee Longsword 1d8+8 or seeker music
Full Attack: +27/+22/+17/+12 Longsword 1d8+8 or seeker music
Special Attacks: Seeker Music (All)
Special Qualities: Darkvision 60 ft., dimension door 1/day, invisibility 1/day, Planar Adaptation, Bardic Music 20/day (countersong, fascinate, inspire courage +1), Bardic Knowledge 2, Rapture of the Song (+2 AC, +2 saves, DR 2/-, freedom of movement), Combine Songs, Subvocalize
Saves: Fort +18, Ref +20, Will +25
Abilities: Str 14, Dex 14, Con 15, Int 16, Wis 17, Cha 30
Skills: Bluff 38, Knowledge (Arcana) 33, Knowledge (The Planes) 33, Listen 33, Perform (Whistle) 40, Spellcraft 33, Spot 33
Feats: Additional Magic Item Space (Goggles), Additional Magic Item Space (Headband), Cleave, Deafening Song, Epic Leadership ^B, Extra Music, Great Cleave, Leadership, Legendary Commander ^B, Power Attack, Skill Focus(Perform[Whistle]), Weapon Focus (Longsword)
Challenge Rating: 27
Possessions: Sandals of the Planes, Bracers of Armor 8, Earrings of Acuity, Earrings of Alacrity, Earrings of Clarity, Earrings of, Earrings of Spirit, Ring of Protection +5, The Starsword, Vest of Resistance +4.
Alignment: NE
Spells per Day: 2
Spells Known: Detect Magic, Mage Hand, Message, Read Magic

Planar Adaptation: Whistler is considered a native of whatever plane she is on as soon as she has spent one day there.

Earrings of Clarity These platinum rings are worn in the lower lobe of each ear and have a diamond set towards the front of each ring. These earrings grant their wearer true seeing. Both rings must be worn for the magic to be effective. This item takes up space as a pair of goggles.
Moderate divination; CL 13; Craft Wondrous Item, true seeing; Price 90,000.
Earrings of Alacrity These plain silver rings are worn in the lower lobe of each ear. These earrings grant their wearer a +2 insight bonus to initiative, the use of evasion as the class feature, and a 10 ft enhancement to speed. Both rings must be worn for the magic to be effective. This item takes up space as a pair of goggles.
Moderate transmutation; CL 10th; Craft Wondrous Item, jump, expeditious retreat;Price 50,000
Earrings of Spirit These rings appear to be made of jade and steel wrapped around each other in a spiral pattern and are worn in the upper lobe of each ear. Earrings of Spirit grant their wearer immunity to mind affecting spells and effects and grant their wearer a +5 luck bonus to saving throws against any spell that would involuntarily separate their spirit from their body, and grants a saving throw for such effects even when one would not normally be allowed, though the wearer can use astral projection and the like normally. Both rings must be worn for the magic to be effective. This item takes up space as a headband.
Strong abjuration; CL 15th; Craft Wondrous Item, mind blank, magic jar; Price 100,000.
Earrings of Acuity These gold rings are etched with tiny runes, and are worn in the upper lobe of each ear. These earrings grant their wearer a +4 enhancement bonus to Charisma, Intelligence, and Wisdom. Both rings must be worn for the magic to be effective. This item takes up space as a headband.
Moderate transmutation; CL 12th; Craft Wondrous Item, fox's cunning,eagle's splendor,owl's wisdom; Price 50,000 gp.
Sandals of the Planes The wearer of these sandals take no penalties or damage from a plane's elemental, energy, or alignment traits.
Moderate abjuration; CL 9th; Craft Wondrous Item, protection from energy, protection from evil, protection from good; Price 12,000.
The Starsword When swung, the blade of this +6 Ghost Touch Longsword appears as a trail of glowing orbs that can be used normally or slung as a full action. When slung, the orbs attack all creatures in a 50 ft. line from the wielder, using the wielder's highest attck bonus with no range penalties, and ignoring any non-force armor. Each orb does 1d8 damage plus the wielder's strength bonus, but power attack can not be applied to these attacks. The blade reforms just before the wielder's next turn, though the wielder is still considered armed when holding just the hilt.
Strong evocation; CL 21; Craft Epic Magic Arms and Armor, meteor swarm, plane shift; Price 128,000.

[Elvaris]

Inhabitants:

Songbirds

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Tiny Outsider
Hit Dice: 2d8
Initiative: +5 (+1 Dex +4 improved initiative)
Speed: fly 30 (poor)(6 squares)
Armor Class: 14 (+2 size, +1 Dex, +1 natural), touch 13, flat-footed 13
Base Attack: +1 Grapple: -9
Attack: Bite (+3 melee, 1d4-1), Screech, or Madden
Full Attack: Bite +3 melee (1d4-1), Screech, or Madden
Space/Reach: 2-1/2ft./0ft.
Special Attacks: Screech, Madden
Special Qualities: Telepathy
Saves: Fort +1, Ref +2, Will +2
Abilities: Str 8, Dex 12, Con 10, Int 14, Wis 12, Cha 16
Skills: Hide 14, Listen 6, Perform(Sing) 8, Spot 6
Feats: Improved Initiative
Environment: Adagio, Layer of the Abyss
Organization: chord (3-10), ensemble (20-40), or chorale(100-200)
Challenge Rating: 2
Treasure: None
Alignment: Chaotic Evil
Advancement: 3-5 HD Small
Level Adjustment: --

This winged creatures looks like a cross between a vulture and an imp.

Once demons of various forms, the songbirds were changed by the primal music into their current forms. They cannot speak but can communicate telepathically over some distance (100 ft.) and so, due to their great numbers, form a massive eyes and ears network across the layer.

Combat
Screech: Every 1d4 turns, three songbirds acting in concert can, as a full round action, produce a blast of noise that does 2d6 points of sonic damage to all non-songbird creatures in a 20 ft. radius. A Fortitude Save (DC 14) halves the damage.

Madden: Six songbirds acting in concert can, as a full round action, amplify their native plane's Maddening song effect, forcing all creatures within 30 ft. of the center of the groups square (All six must be in the same 5 foot square) to make a save against the effect as if they had spent a full day exposed to the song. For every two birds over the required six, the DC of this save increases by one. Any individual songbird can use this ability twice per day.

Tuneless

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"Tuneless" is an acquired template that can be added to any creature (referred to hereafter as the base creature).
Size and Type: The creature's type changes to Outsider (Extraplanar, Evil). Do not recalculate base attack bonus, saves, or skill points. Size is unchanged.
Speed The base creature's speed is reduced by 10 ft.
Attack: A tuneless creature retains all the attacks of the base creature and also gains a slam attack if it didn't already have one.
Special Qualities: A tuneless creature loses any spellcasting or spell-like abilities of the base creature as it cannot speak, but gains those below.
Awkward Grace: Despite their generally slow and sluggish movements, a tuneless creature moves very rhythmically, in tune with the song of their plane. As they tend to move in unison, tuneless creatures get a +1 bonus to all attack rolls for each three other tuneless creatures within 50 ft.
Thrall: A tuneless creature can be controlled by any Seeker of the Song who succeeds at a DC 20 class level check. If two Seekers are attempting to control the same creature, or one attempts to break the control of another Seeker, the outcome is determined by an opposed class level check. A Seeker can directly control a number of HD of tuneless creatures equal to twice their class level.
Empathy: A tuneless creature can communicate basic emotions (fear, danger, surprise) to any other native of their layer within 50 ft.
Abilities: Same as the base creature, except that a tuneless creature has no Wisdom score, and its Dexterity drops by 2.
Skills: Tuneless lose the ability to speak. Otherwise same as base creature.
Environment: Adagio, Layer of the Abyss.
Organization: Solitary, mob (4-12), or horde (15-30)
Challenge Rating: As base creature.
Treasure: None.
Alignment: Any Evil.
Advancement: As base creature.

A tuneless creature appears as the base creature, but with a gray oall to their coloration, and a distant look on whatever features they may have. They move like marionettes, all in time to some cosmic music, but if they ever leave a place where that music is dominant, they collapse as if their strings had been cut and die instantly.
Sample Tuneless:

Tuneless Quasit
Tiny Outsider (Extraplanar, Evil)
Hit Dice: 3d8 (13 hp)
Initiative: +6
Speed: 10 ft. (2 squares), fly 40 ft. (perfect)
Armor Class: 17 (+2 size, +2 Dex, +3 natural), touch 15, flat-footed 14
Base Attack: +3 Grapple: -6
Attack: Claw +8 melee (1d3-1 plus poison)
Full Attack: 2 claws +8 melee (1d3-1 plus poison) and bite +3 melee (1d4-1)
Space/Reach: 2½ ft./0 ft.
Special Attacks: Poison.
Special Qualities: Alternate form, awkward grace, damage reduction 5/cold iron or good, darkvision 60 ft., empathy, fast healing 2, immunity to poison, resistance to fire 10, thrall.
Saves: Fort +3, Ref +5, Will +4
Abilities: Str 8, Dex 17, Con 10, Int 10, Wis --, Cha 10
Skills: Bluff +6, Diplomacy +2, Disguise +0 (+2 acting), Hide +16, Intimidate +2, Knowledge (any one) +6, Listen +7, Move Silently +8, Search +6, Spellcraft +6, Spot +6
Feats: Improved Initiative, Weapon Finesse
Environment: Adagio, Layer of the Abyss.
Organization: Solitary, mob (4-12), or horde (15-30)
Challenge Rating: 2
Treasure: None
Alignment: Chaotic evil
Advancement: 4-6 HD (Tiny)
Level Adjustment: —

Poison (Ex)
Injury, Fortitude DC 13, initial damage 1d4 Dex, secondary damage 2d4 Dex. The save DC is Constitution-based and includes a +2 racial bonus.

Alternate Form (Su)
A quasit can assume another form at will as a standard action. Each quasit can assume one or two forms from the following list: bat, Small or Medium monstrous centipede, toad, and wolf.

[Elvaris]

My selection of books is somewhat limited and doesn't include the Epic Level Handbook or any of the Planar Handbooks outside of the DMG and SRD, so I was flying a bit blind through some of this.