Sneaking carefully through the castle, Kain made sure to check every single corner. It was night now and everyone should've been in bed. But he was still cautious. He didn't need anyone knowing that he had snuck out of the castle to feed.
There was an eerie silence as he tiptoed through the halls. The only light coming from the odd wall lantern here and there and the moonlight shining through the windows.
He moved silently, stealthily, being sure to tread as light as possible so no one would be able to hear his footsteps. The one thing he was dreading was seeing Arden in the morning. He hadn't told his sister that he was going out for a few hours to feed, since she was hauled up in the Captain's office with the short man himself.
Now he knew that he was most likely going to receive another slap to the back of the head. Just thinking about the painful action made him whince and subconsciously rub the exact spot she always managed to hit.
Unlike his older sister, Kain was known to indulge slightly whenever he felt hungry. Whilst Arden would always put off feeding until the last possible moment, Kain would happily feed at the slightest pang of hunger. Sometimes just feeding because he was bored or felt like it.
When a demon feeds on a human, they not only take the delicious, red liquid out, but can simultaneously deposit a toxin into the victims bloodstream, should the demon choose to do so. The toxin comes from the demons sharp, fang-like canines. The toxin induces a temporary state of delirium: meaning the victim becomes disoriented and suffers from memory loss. In other words, the toxin makes the victim easy to feed from whilst ensuring they won't remember that they became demon fodder.
Due to Arden's wishes, whenever the two fed they would always be sure to injection the toxin into the victims bloodstream so they couldn't remember being fed from. Ofcourse, Kain had suggested multiple times to just drain the human of their blood and kill them to make life simpler. But as Arden stated, there is already so little of humanity left, they shouldn't die if they can live.
A few times, the siblings had fed from titans. But with how quickly Titan blood evaporates its not always fulfilling to drink their blood.
Opening the door to his bedroom was extremely painful for Kain. He remembered how it creaked the first time he opened it and definitely wasn't going to let it be so loud when he was being sneaky. Tobias, who he shared the room with, was a heavy sleeper for someone so timid and jumpy. So even if he made a little noise, Kain knew it wouldn't be too much of a problem.
Sliding inside the room, Kain swiftly closed the door infront of him. Slowing down his pace as the door made a satisfying 'click' sound. Sighing in relief, he grinned goofily before resting his forehead on the hard, cold wood of the door.
"Operation: Sneak in. Successful." He quietly whispered in victory. Before turning around and letting out a loud gasp mixed with a shrill scream.
Only centimetres from his face was a pale, slender hand with long, black nails. The nails were at least two inches long and came to deathly sharp points. They were more like talons rather than nails, except they didn't have such a prominent curve.
But what caught his attention was the thick black blob clutched in the slender hand. The talon-like nails were digging into the squishy surface as it squirmed and wriggled in in the clutches of the hand, a thick black liquid oozing through the holes that were pierced in its skin. The blob was wrinkled and had two great big beady eyes, fiery orange in colour and the whites of the eyes were a deep red, not white. Its mouth was a small circle filled with multiple rows of razor sharp teeth.
A lesser demon.
The weakest of the demon kind. They tend to manifest as small blobs like the one Arden was holding, or humanoid creatures that appear as shadows. Their bodies are deformed and disfigured. Though they are the most inferior of the demon race, they are still extremely deadly. Most draw their power from their masters, the greater demons.
Greater demons.
The most powerful and frightening of the demon race. They appear perfectly as ordinary humans. But their power surpasses that of any force currently existing in the human realm. There aren't many greater demons who willingly travel to the human realm. Instead, they send their lesser demon slaves to do all their mad bidding.
"It seems you have a tail." Came the calm voice of the owner of the talon like nails.
"W-what?" Kain questioned, his body pushed against the door in both surprise and fear.
Looking behind the hand, Kain came face to face with long, glossy raven black hair. Blood red, crimson eyes that held no emotion but were slightly narrowed. And a blank, expressionless face.
Kain immediately relaxed when he realised that it was just his older sister, and not an intruder.
Arden kept the wriggling demon blob infront of Kain's face as she spoke, a displeased edge to her voice, "What's the point in having the ability to detect the presence of other life forms, especially demons, if you aren't going to use it?"
"There's too many people in this castle, it's overwhelming! Not every demon is a master of their abilities like you, ya know?" He mocked uncomfortably as the demon blob squirmed only an inch from his face. Its black blood now dripping down Arden's talons, along with her fingers.
"If you actually practiced when I was teaching you, then I wouldn't have been able to sneak up on you. Or this little guy." She lectured coldly, nodding her head at the lesser demon still clutched tightly in her right hand.
Kain raised his hands in surrender whilst pleading with his sister, "Alright, alright! Just move the ugly bastard from my face already." He whined.
Crushing the demon blob in her hands, Kain cringed at the loud squelching sound of its body as it was killed. Cold steam rose from Arden's hand as the demon blobs body, a long with its blood, evaporated into thick, black smoke.
When she opened her taloned hand, a small glowing ball of white, yellow and purple illuminated her palm. Mist of the same colours twirled around the ball. It was only about the size of a marble and the colours inside swirled beautifully. Bringing her hand to her mouth, she swiftly swallowed the small glowing ball, humming in satisfaction.
A soul.
That's what the soul of all living creatures looks like. Just a small, marble sized ball of glowing light. The other way a demon can feed is by eating the souls of the victims it kills. However, human souls are very rarely satisfactory for a demon. Most demons prefer the souls of their own kind as they increase their power.
Looking around, Kain noticed the absence of his roommate, "Where's Tobias?" He questioned.
Folding her arms over her chest, Arden spoke calmly, "I told him to stay in my room tonight whilst I waited for my idiot brother to come back to base."
"Heh, about that--" Kain started with a goofy grin before a rough slap connected with the back of his head. He yelped in surprise and pain, "Ow! Just when I thought I was safe." He whined whilst rubbing the tender spot on the back of his head.
Sighing in frustration, Kain folded his arms over his chest, "What's a lesser demon doing following me anyway?"
Arden's talons extracted back until she was left with perfectly normal, human nails. After having examined the lesser demon in her hand before she crushed it, she could assess one thing. "Drystan Daine." Came her simple reply as she locked eyes with her younger brother, a now serious expression on her face.
Kain's arms dropped to his sides, his eyes widened in disbelief at the mention of the name, "The supplier?" He questioned though he already knew the answer.
Walking over to the window in the room, Arden glanced around carefully. Her blood red eyes illuminating in the moonlight as she spoke coldly, "It seems the greater demon wants to talk."
~•~
This is the closest thing on Google I could find that matched my description. Just imagine it with the colours I described or don't if you don't want to. It's just to help you visualise.
For the other lesser demons, the 'humanoid, disfigured and deformed ones' just imagine a dementor from Harry Potter but it's limbs are disproportionate and messed up.