Those fallen who are in possession of bodies will gain far fewer powers than the canonical World of Darkness versions. Each fallen will gain a single power that has greater versatility, such as pyrokinesis, to encourage the players to think in terms of a defining power rather than a random assortment of magic.
The Fallen will have an extra ability based on their full demonic form which holds true for all members of their House and two abilities that are unique to their particular purpose and sub-House. Using their demonic form can draw the attention of local supernatural forces and can sometimes cause them more trouble than its worth. It costs a point of Faith to use one's revelatory / apocalyptic form but it lasts until the Fallen wills it to end. So long as they are wearing their apocalyptic form, they may suppress or re-establish their active abilities with a thought. As an example, they may begin with wings, suppress them to move through a hallway, then unleash them again to fly over a gap.
They may use rituals, much as anyone else, but doing so typically causes their minds to brush the greater cosmos or even channel the power of Earthbound and Outer Gods, which is still stability shaking to them. They may spend Faith instead of Stability on a 1 for 1 basis when using spells or rituals as part of the activation cost.
Some things may become normalised to them with exposure and thus after each exposure the possible Stability cap will be reduced by 1 for each independent exposure. This works twice as quickly for Deep Ones and Zombies which even humans can become inured to (not typical of the game but there are enough examples in the books of people being disturbed by the monsters but no longer falling into hysterics).
They still need to worry about losing sanity as they come to comprehend how the universe truly works - unravelling their human comprehension removes the bulwark of protection against the torments of the abyss, after all.
When the Fallen die, they can potentially claim new Hosts. The players have given me the power to simply provide them with a logical host after the first which I'll draw from the Masks of Nyarlathotep companion book or other NPC guides depending on the country they're in and the campaign they're playing.
Fallen heal faster than mortals. Thus they heal 1 hit point per day without rest, 2 health points per restful day and 3 health points if given medical attention. They can spend a point of faith to undo an injury (i.e. broken limb or lost finger) or regain health points on a three health points for one faith point basis. Fallen also won't lose their last health point from bleeding out.
The Fallen can also invoke each other using their celestial name which allows them to converse through whispers from across a field as though they were standing beside each other. Once can detect this power's use through the Awareness skill. Mortals can also invoke them with their celestial name but this is a one-way street.
It is a +3 stability cost to witness a demon's apocalyptic form and their reactions will be dependent on the Fallen's overall behaviour toward humanity. A Fallen Angel who slaughters humanity with relish will provoke terror, while one who seeks to protect humans will typically leave them awestruck and astounded. The cost will be reduced by one point for every encounter until the third encounter, whereupon the mortal will lose 1 point of sanity but no longer lose stability to gaze upon them as their mind has come to accept them. After all, they are creatures that helped stabilise reality so they do not appear as "wrong" as most.
Thralls only take a hit to their Stability the first time they see the Fallen.
Fallen angels may refresh their native faith pool (i.e. equal to their normal Faith rating) through the quiet worship provided by mortals but only if they have recently done something meaningful for that mortal (i.e. save their life, defend them in apocalyptic form, torture them). This is called 'reaping faith' and it can be performed on a thrall if the situation is right. Typically they may only do this once per case though exceptional circumstances may allow additional refreshes.
Additional SkillsFaith is a General Skill that is capped at 3 for starting characters. Fallen may gain double their initial rating through pacts (i.e. temporarily increasing it to six) on a one Faith per pact basis. There may be some occasions where a Fallen needs to roll their Faith rating as a contested pool though typically it is simply spent to perform an action. Fallen recharge one Faith per day and must reap from mortals (see last paragraph in Base Abilities) or ravage their thralls to regain more (see Pacts below).
Awareness is a General Skill that is used as a sort of sixth sense for the supernatural that can sometimes glean further information, but is typically used as a divining rod for relics and current magical effects.
Eminence is an Investigative / Interpersonal Skill that is used as a sort of Credit Rating skill for the fallen. There is also a downside in that other Fallen are more likely to remember their names and potentially even syllables of their True Name.
Legacy is an Investigative skill where Fallen may spend points to remember situations and events from before their time in the Abyss. Most Fallen would have at least one point in this skill. Typically spends will lead to short flashbacks.
In order to better reflect these skills, I have used a 28 / 75 point system. Yes, these are a lot of points but in a two-player game with additional skill requirements I don't see this as a major hindrance.
One can use their True Name to bind them to perform a certain number of actions to fulfil the sorcerer's will. Earthbound can bind them forever if their True Name is known or even consume their souls should their hosts be broken before one. Fallen can be banished or abjured against through mortal ritual and certain ritualistically enhanced items affect them more keenly than any mortals. They cannot succumb to mundane disease nor do they age, but they do need to eat, sleep and breathe like any mortal. Elder Signs are particularly effective repositories of human faith (i.e. psychic energy) as they are already powerful occult matrices. They don't need to see the Elder Sign, or know it's there, to be affected by it. On the other hand, they can easily approach should they come across a neglected Elder Sign displayed prominently.
The Fallen can make a pact with a mortal in exchange for more Faith. Each mortal in a pact provides an additional point of temporary Faith but the Fallen may only have as many pacts as their Faith rating. The Fallen may ravage the mortal for more faith (rolling his own Faith rating to see how much is gained) but this costs the mortal +7 stability and permanently reduces both their current and maximum sanity by two.
A mortal (aka thrall) under the effects of a pact ceases to age or be affected by the rigors of age (including rigors currently experienced), and gain three points in an investigative skill and five points in a general skill (may not be Sanity or Stability). This skill may be Awareness and, indeed, is typically the only way such individuals may obtain that skill. The Fallen can roll their Faith to sense the mortal's location, no matter where they are.
There is also a once off benefit at the point when the pact is established where the Fallen may either refresh all of the mortal's hit points to full while curing them of a disease or injury, or provide them with apparent youth and beauty, or provide them with 3 sanity points (as they remind humanity of their value in the world) or grant them the use of an apocalyptic form trait known to the master which costs the thrall -2 stability to use.
The Fallen may also possess the thrall at will should they ever be ejected from their host. They can also open up their thrall to possession by another, but only if the thrall's current stability or sanity has been reduced to 0 or they are unconscious.