Fiend

Fiends are creatures of wickedness from the lower planes. Devil are usually native to The Hells. Demon are from The Abyss - a chaotic evil universe unto itself with uncountable layers of infinite variety connected haphazardly. Some fiend reside within the Underdark of Ru'Mal and Ru'Than.

A few fiends are the servants of deities, but many more labour under the leadership of archdevils and Demon princes. Evil priests and mages of Esau may sometimes summon fiends to the material world to do their bidding - which is a dangerous undertaking, as they will turn on their 'master' quickly.
If an evil celestial is a rarity, a good fiend is quite inconceivable.

DEALS

Entering into any fiendish bargain, especially with a Devil or a Demon, requires certain knowledge that the people has what it takes to decimate the foul creature if necessary. Anything less is just a foolish moment closer to a dismal end.

Cheating to win should be a standard operating procedure for any fiend. They will lie, cheat, steal, bully, torture, or kill to get what they want.
Weapons usually are barbed or poisoned. While destroying the party outright is always an option, anything that causes pain and suffering is open for consideration. Opportunities to burn or crush the people they are dealing with could be introduced with traps or terrain effects.
Being savaged by dire animal companions adds another dimension to encounters, keeping anyone busy with minions while the fiends focus on making an example of any mages. Capturing foes for later interrogation and torture is another great way to impress upon people the truly capricious evil of these beings. Tearing in mind, fiends are not simply powerful monsters; they have goals to destroy all that is good and heroic in the world. They want to infest the mortal plane with their own depraved wickedness. “I want to see the sun blotted out from the sky” is a sentiment that many fiends would find intriguing.

Casual Cruelty

Fiend are evil and often have no regrets for those they grind up in their wicked ambitions. They laugh at the piteous cries of their captives and spit in the face of any who attempt to parley or derail them from their goals.

Depending on the fiend’s sphere of interest and alignment, they may use psychological warfare to confuse opponents or attack loved ones in the person’s life.

Conceive intricate plans and ways to lead people around, testing, provoking, and mentally grinding the heroes down. Harry a person of weaker character constantly in the sure knowledge that their victim can do little to respond. Wait patiently until a particular person is on watch and make a lightning attack only to disappear while the rest of the party is rousing from slumber. They might do this for nights on end if the group cannot find a way to secure their camp against it. The affected person check might risk fear taking hold and their sanity begins to erode. All along becoming exhausted from lack of a good rest.

Physical attacks might conclude with horrifying results. Tearing and rending, wounds that heal with grotesque scars, punctures that constantly itch, and diseases or secondary effects that cause the skin to crawl, or quiver, or have some other reaction are all excellent ways of separating fiends from other tough, supernatural foes. A fiend’s touch bears the vileness of the infernal and diabolic realms they come from.

True Name

Every creature has a true name that makes it entirely possible for other creatures who know it to gain control over them. For fiends, their true name is a double-edged sword. When their true name is known it makes it possible for a fiend to rise up the fiendish ranks - something that can be very difficult for fiends without that knowledge. Yet, on the other hand, if knowledge of a fiend's true name somehow was discovered by a person, another fiend, or another creature, it will grant that creature control over the fiend. This control extends very far.
For example, by a mortal knowing the true name of a fiend, then the mortal can call the fiend from the Lower Planes to the Prime Material Plane, and also force it into servitude. Fiends can inconvenience or torment their rivals by helping a mortal to discover their rival's true name.

As a fiend needs knowledge about its true name to rise up the fiendish hierarchy - it often happens that their true name becomes in different, changing forms. They might collect titles and rankings. It is believed that this is not the true name changing instead some form of evolution of the true name. The core of the true name remains the same, but there are some additions to it. When a mortal uses an outdated true name to try to bind a fiend, the fiend responds to the binding. However, the control the mortal exhibited over the fiend is not complete, allowing the fiend to create fatal consequences for the mortal.

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Types of Fiends

Arcane Devourer
Archdemon
Burning Dervish
Cackler
Cambions
Chaos Knight
Clockwork Abomination
Daemon
Demon
Demonic Knight
Death Wing
Devil
Devra
Draegloth
Dread
Dream Eater
Dream Spectre
Dream Stalker
Eremite
Faceless
Fallen Angel
Hell Hounds
Ghul
Gloom Demon
Grendel
Mandrake
Night Hags
Nightmare
Nuckelavee
Onaryx
Phase Hounds
Rakshasas
Shadow Mastiff
Sihuanaba
Silvertongue
Simpering Maggot
Sin Seeker
Snow Terror
Soul Eater
Succubus/Incubus
Swordspirit
Tardigrade
Udug
Ukradolo
Ultar
Vexxeh
Wanyudo
Xydragis
Yugoloths