Café de Nuit

Van Richten's Guide to Fiends has a list of power rituals that got left out of the omnibus version. It has the powers gained by fiends that successfully complete rituals in Barovia, Bluetspur, Borca, Darkon, Dominia, G'Henna, Har'Akir, Hazlan, Invidia, Kartakass, Keening, Lamordia, the Nightmare Lands, Sithicus, and Sri Raji.

That leaves plenty of lands that don't have explicitly defined power rituals, ranging from domains that no longer exist (Gundarak, Arkandale) to those that only briefly existed (Cavitius), but also has some surprisingly major gaps like Richemulot and Falkovnia. Have people come up with ideas for powers from the other domains or, if not, can we start brainstorming potential powers that are different from what already exist to allow even more variety in the fiends of Ravenloft?

I think the Kargatane had published some more in one of their BoS series.

I'd like to help... After I read 'Van Richten's Guide to Fiends'. And learn 2e mechanics.

Falkovnia is out? That is surprising as I remember there was a succubus from there, but I may remember wrong.
Meanwhile, I suggest this:
Do not feel too constrained by "this power for this domain". I was always looking for appropriate powers, whether they were the official one or something similar. After all, fiends do different rituals to gain different abilities. Especially in 2e, even the size of their Reality Wrinkle was different from fiend to fiend (there was no 2000 feet per HD, as I recall).

Falkovnia (Rage): Once per day, the fiend can enter a rage, as a 15th level barbarian, for 1 rnd per hit die.

Children of the Night: Demons by the Kargatane has Land Based Powers for fiends from the following domains.
For more details download https://www.kargatane.com/sotk/netbook/cotndpdf.zip from https://www.kargatane.com/

I made some alterations based on CotNDemons v8
Unless otherwise noted all land-based powers are considered supernatural powers

The Core

Barovia (Obscuring Mist)
Blaustein (Domination)
Borca (Poison Touch)
Darkon (Forget)
Dementlieu (Mass Charm)
Demise (Petrification Gaze)
Dominia (Cause Insanity)
Falkovnia (Rage)
Forlorn (Time Shift)
Hazlan (Detect Magic)
Invidia (Corrupt Life)
Isle of Ravens (Fly)
Kartakass (Siren Song)
Keening (Wail of the Banshee)
Lamordia (Hyper-regeneration)
L’ile de la Tempete (Hypnotism)
Liffe (Incompetence)
Markovia (Create Broken One)
Mordent (Phantom Shift)
Necropolis (Slay Living)
Nocturnal Sea/Nebligtode (Dominate Undead)
Nova Vaasa (Inner Turmoil)
Richemulot (Dominate Animals)
Sea of Sorrows (Geas)
Shadow Rift (Deeper Darkness)
Sithicus, Lord Soth (Shadowalk)
Sithicus, Inza (Shadowform)
Tepest (Spell-like Abilities)
Valachan (Polymorph Touch)
Vechor (Alter Reality)
Verbrek (Mass Polymorph)

Islands of Terror

Bluetspur (Mindblast)
G’Henna (Cause Hunger)
Kalidnay (Desiccate)
Nightmare Lands (Control Dreams)
Nosos (Putrefy)
Odiare (Improved Possession)
Rokushima Taiyoo (Cause Strife)
Scaena (Major Image)
Souragne (Animate Dead)
Staunton Bluffs (Ethereal Jaunt)

Clusters

The Amber Wastes
Har’Akir (Disease Touch)
Pharazia (Face of Evil)
Sebua (Detect Life)

The Shadowlands
Avonleigh (Invisibility)
Nidala (Conversion)
Shadowborn Manor (Animate Objects)

The Verdurous Lands
Saragoss (Instil Paranoia)
Wildlands (Steal Abilities)
Sri Raji (Phantasmal Killer) /(Fear gaze)

The Frozen Reaches
Sanguinia (Drain Humors)
Vorostokov (Alter Temperature)

Zherisia
Paridon (Aura of Terror)
Timor (Compression)

2e domains missing are Arkandale, Gundarak, Dorvinia, Ghastria, I ' Cath, Daglan, House of Lament and finally Cavitius and Tovag from the Burning Peaks Cluster.

My suggestions are change Falkovnia to (Spell Resistance)

Arkandale (Conjure Animals)
Gundarak (Bloodlust from Prayers from the Faithful p.16)
Dorvinia (Poisonous Balm)
Ghastria (Trap the Soul)
The Drowning Deep (Implosion)
I' Cath (Dream Reality)
Daglan (Create Goblyn)
Cavitius (Energy Drain)
Tovag (Horrifying Visage)
House of Lament (Passwall)