Alignment Synchronizing
Because all darkness and all light each originate from single points (these being Saten and Yuhel), all beings of an alignment are connected to all other beings of their respective alignment. This creates the very strange artifact of damage synchronizing.
Basically, if an angel of darkness attacks another angel of darkness, all physical damage that they inflict upon each other is mimicked between them. This means that if one scratches the other, the scratch appears on their own skin as well; if one cuts off the arm of the other, well, their own arm is coming off, too. This works regardless of the power involved. If a low-level angel uses all their power on a high-level angel in an attack that would kill any angel on their own level, and only inflicts a scratch, then only a scratch is reciprocated. Similarly, if a high-level angel uses only a bit of their power and badly injures a low-level one, they receive identical injuries even though the same attack used on themselves would cause barely a scratch.
What counts as "inflicting damage" is vague and sometimes arbitrary. Inflicting injury on oneself during a fight in the hopes of sending the same damage to the opponent definitely does not work. However, rushing headlong into an easily avoidable attack in order to reciprocate the damage does. Setting traps to inflict damage later is where things start to get complicated, and will be handled on a case-by-case basis should it come up in RP.
Yuhel and Saten are the only ones able to negate this effect, and this is done by physically canceling it out with a method that leaves a permanent mark on the one who is being removed from the synchronizing. They don't do it often, and Saten will almost certainly refuse any requests. To note, this is officially only used in the case of executions, however there is currently at least one angel (who is of light alignment) who is currently living and has been de-synchronized.
In a panfandom setting, Saten's synchronization would carry over to any being who possesses powers that would be considered "darkness" and no powers which would be considered "light". Any character possessing mixed power would likely be exempt. However, given that the binary in her world is beginning to break down, there do exist those who are synchronized with both light and darkness, though they are rare and inflicted damage tends to be reduced some upon reciprocation. There is also one known pair of angels whose alignments change with the phases of the moon (literally; they're on a lunar cycle) so if a character switches back and forth between "light" and "darkness", they might possibly be synchronized only part of the time.
For this, keep in mind that evil is not necessarily darkness, and good is not necessarily light. Elements do not qualify on their own unless there is a clear alignment to them (for example, "dark fire" would count towards darkness, but simply "fire" would not). Healing by itself is not light, per se, but if it's explicitly stated to be holy power (and this is not contradicted by surprise plot twist later), then it would probably count.
As a final note, because Saten is the strongest form of pure darkness, her presence causes a suffocating feeling for those light-aligned, which can range from slight discomfort if they're strong (or not very heavily aligned) up to actual physical sickness if they're on the weaker side. If you don't want your character to deal with this even though they fit the definition of light-aligned, feel free to say that their world just doesn't synchronize properly with hers.
Forced Synchronization
The specialized power of angels of darkness is a power that can most easily be referred to as mind control, although this is not an accurate description of what it does. Simply stated, this power forces a close synchronization between the user and the target, and inflicts the user's will upon the target. While this power is in use, any damage dealt to the target is received by the user regardless of their own personal alignments or the alignment of the attacker; although this means that if an angel of darkness is doing the attacking, they too will receive damage due to inflicting damage upon another angel of darkness, even though there's a degree of separation between them. As well, any damage dealt to the user is also received by the target, which would likewise damage an angel of light if another angel of light was currently being controlled. This bond between user and target overrides all alignments, including a lack thereof.
In addition, if the user's will is on an equal level with that of their target, they can prevent the target from taking any course of action. If their will is stronger, depending on the degree of strength, they can do from small movements to outright full control over their target, though the best they can do as far as actually controlling a mind is to shut it down temporarily; otherwise the target is fully aware of what their body is doing even if they can't give any sort of reaction to it. In this state, utilizing a target's powers (if unfamiliar to the user) is next to impossible, except for those which are triggered by purely physical motions. If the user knows how to use a target's power, it becomes possible, though far from trivial, to activate it themselves.
And now for the handy text box!
Character: Username:
Is this character specifically aligned to light or to darkness as defined in Saten's world? Which?
If yes to the previous, do you want to allow this alignment to synchronize with Saten?
(For light, this would mean her presence causes some level of discomfort. Additionally, if this character is very strong themselves, she'll also feel the same effect to a minor degree.)
Would you be willing to have Saten force synchronization with this character at some point, assuming that we worked out the details ahead of time?
Note that filling this out is completely optional, and if you don't, I'll just assume that alignment doesn't take effect on your character unless I feel like specifically asking you.