The user is able to enter a state which allows demon-like abilities, skills, and traits with demonic capabilities and powers that transcend all of the user's basic limits whether physical, spiritual, mental,and so on.
Desperado (Rakudai Kishi no Cavalry) by using Excessive Awakening
Amira (Shingeki no Bahamut)
Hendrickson (Nanatsu no Taiza); via demon's blood
Shiki Granbell (Edens Zero); Overdrive
Allen Francis Doyle (Angel) can freely shift into a demon mode due to his half-Brachen demon heritage.
Rodin (Bayonetta) in his demonic form, Rodin the Infinite One.
Whistler (Buffy the Vampire Slayer) in his demon form.
Belthazor (Charmed), Cole Turner's demon mode.
Chrono (Chrono Crusade) in his demon form.
When he was stabbed by his Rebellion, Dante (Devil May Cry series) became able to access to his Devil Trigger. With it he is able to assume his full demonic form, gaining an increase in strength, strength, demonic power, and healing factor.
When he embraced his demonic heritage, Vergil (Devil May Cry series) became able to access his Devil Trigger. With it he is able to assume his full demonic form, gaining an increase in strength, strength, demonic power, and healing factor.
When he came in contact with the Yamato, Nero (Devil May Cry series) was able to access his Devil Trigger. However, instead of a demonic form, Nero gains a translucent shadow of a demon that fights as he does while still retaining the increase in strength, speed, and healing factor. Even so...
...he eventually gained his true, demonic form through his desire to reconcile his revealed uncle and father, Dante and Vergil.
Ash Williams (Evil Dead: Regeneration) can transform into a powerful deadite, Bad Ash, by absorbing demonic energy from Deadites.
InuYasha (InuYasha) transforms into a full yōkai upon losing Tessaiga.
Rikuo Nura (Nura: Rise of the Yokai Clan) becomes a powerful demon during the night.
Yusuke Urameshi (Yu Yu Hakusho) in his demonic form.
After absorbing his Ghost Chess henchmen, Kapellmeister (Marchen Awakens Romance) activated his strongest Ghost ÄRM, turning him into a demon.
By abandoning her ego as a human, Nene Saikyo (Rakudai Kishi no Cavalry) can access the "Excessive Awakening" a form that lets her turn into a Demon gaining an enormous boost in power.
Naseem Al-Salem (Rakudai Kishi no Cavalry) can use a rare form among other Desperado, letting him become more like a demon granting him an enormous boost in power.