Demons and their kin communicate through a mix of body language, fragmentary snippets of a forgotten language, and by a limited form of telepathy that consists of "pinging" each other's auras. The latter most aspect is the most important due to many demons lacking the means to either see or hear.
When a demon wishes to convey a message, they radiate a pulse of "intent" that is recieved by any nearby demons upon colliding with their personal auras. The demon sending the message will usually accompany this pulse with a verbal sound but that is not necessary.
Upon recieving the message, the demon being spoken to will then experience a physical sensation accompanied by a flash of mental imagery.
The physical sensation's intensity and nature is determined by the speaker's emotional state and intent. A respectful greeting may feel like a light vibration while a friendly compliment might induce a pleasant tingling upon the back of the neck. A fierce rebuke or threat on the other hand might feel like a bone-shaking shockwave that radiates throughout the body. A message from a speaker that is emotionally conflicted can induce two very different sensations at once.
he mental imagery is conjured by the recipient's mind as it tries to interpret the message being given. This imagery is often crude and straight-forward, using only concepts the recipient understands. Thus the reciever's mental image is roughly in-sync with the speaker's meaning up to a point.
Example - if Demon A tried to tell Demon B about an ocean, and Demon B has never encountered an ocean. Demon B however has experienced a desert and the phenomenon of rain. Rather than seeing the ocean, Demon B's mind would visually interpret the message as "dunes of rainwater".
A demon can lie by sending false emotional impressions or 'forcing' the meaning of their message. This is something that usually has to be trained in order to be effective, as a false message can read as hollow or off-puttingly forced. In the same way a smile can seem forced, the emotional intensity of an deceptive message can feel "off". Especially if the parties in question know each other well.
From birth every demon carries an innate but fragmentary knowledge of progenitor's language. Believed to be the language of the divine, this language is often spoken in tandem with the telepathic messaging in order to provide better clarity. Due to their nature as mere twisted fragments of divine essence, demons have only a very rough grasp on this language, with an individual demon probably only knowing five or six words at birth. They can of course learn more by meeting other demons and pooling knowledge together, but as large congregations of demonkind never last long before imploding, every attempt at creating a widespread lexicon has failed.
Demons can communicate via aura with any species capable of exhuding or percieving an aura or possessing a soul. Unless the being makes a conscious effort to train however, their messages will appear crude and confusing. Their messages might come off as too intense one moment and frustratingly subtle the next. This fact combined with the tendency for demons to be emotionally volatile often leads to conflict.