There were a few options here. In order of least desirable to most, they were as follows: He could not learn the demon wing transformation and risk his life trying to find a way up, he could use the considerable amount of life force in his staff, likely depleting it a great amount, or he could kill one of his slaves (probably the large one to be safe) and use all of that life force to power his spell. The problem with his master teaching him is that he has no time to experiment and find that perfect balance that uses the least power. Oh well, if wishes were fishes... You'd have a massive pile of useless, smelly fish
"Fish that I could use to power this spell of yours"
True enough
"Anyway, as I was thinking, there is a certain setback that will arise from killing one of my slaves."
Do tell. The intricacies of human interaction interest me to no end...... not
Ignoring his master's sarcasm, Reaper continued"Well, if I kill one of my slaves, then the other will surely think, not incorrectly, that he shall be meeting the same fate. I will need to find a way to keep him in place, preferably without something as energy consuming as a conditional death spell." pausing to think for a moment, an idea came upon him. "I could fake it though."
You could what? In my experience, which is far greater than yours mind you, I find a spell only works if you DON'T fake it.
"As you and I have both seen, these slaves are both gullible and superstitious. There is no reason to truly cast the spell, as long as they believe it is cast. Their fear of a slow excruciating death followed by eternal damnation will surely be enough to keep the twig I'm allowing to live in place."
The Puppet Master scoffed,As if even something as foolish as a slave would believe you have the power to send them to hell
At this Reaper smiled. "I assure you, by the time I'm done with the large one, he will believe I myself am a demon and don't merely have one within me. Now teach me the spell and we can return to camp, so that I can show you my plan instead of merely telling you about it"

After learning precisely what he was to do, Reaper was confident that the life of the slave would be enough to maintain the spell for up to a day, though probably slightly less. He could also bolster it with other life before it ended if need be. His primary concern was how long it would take him to learn to use the whole new set of muscles he was adding, but The Puppet Master has offered to provide that muscle memory for him. With how forthcoming his master has been being lately, Reaper can't help but wonder if his master has a hidden plan for him that is nearing fruition. Or perhaps he is merely finally seeing Reaper's true power and potential, unlikely but possible.

With the spell learned, all that was left was to enter camp and deal with the slaves. It would be easiest to kill the other in his sleep, cast the spell, then wake up the other and fake the conditional death spell. The slaves were looking tired as he entered, so he told them to get right to sleeping. The sooner they were soundly asleep, the better. Once Reaper could be sure they were asleep, he went over to his bag and gently pulled out a black leather knife carrier. Each blade had a perfectly sized sheathe of soft but strong leather. These were his masterpieces of his human research. Each blade magically imbued with the power to do something to the human body. His fingers danced over the unique, silver-cast handles that protruded until he came to the one with a skull at the base. Slowly removing it from its sheathe, he carefully caressed the blade. Although the most simple of his blades, this one was his favorite. A foot long, thin, and sharp enough to pierce bone, it glowed softly in the glow of the moon overhead. This needed only to pierce a human heart and the person would die instantly. Not good if you wanted any of the organs to remain intact, but very effective for simply killing.

Shaking himself out of his reverie, Reaper crossed the camp to where the slaves slept a few feet apart. He smiled. Any normal assassin would have trouble not waking the other with his murder, but he was no normal assassin. Kneeling on the side opposite the other slave, he swiftly plunged the dagger into his victim's heart. His eyes flashed open pain showed briefly on his face, then he was in death's frozen grasp. Acting quickly, Reaper sat cross-legged on the dew covered grass a few feet away. Briefly focusing his thoughts into a precise, sharp tool like his bloodied dagger by his side, he drew out the life force freshly freed from the corpse of his slave. Maintaining his focus, he drew the life into himself and set about bringing his master's spell to life. he felt the tug and itch of new growth, followed by the pain of his own muscles being rearranged to make room for the massive wing muscles to sprout out. The was a slow ripping sound as the wings grew from his back and pushed themselves from his robe. As his wings continued to grow, Reaper threw in some horns on his forehead for effect to better scare his slave. At least that was what he told himself. As the pain of the transformation subsided, he felt a new weight on his back that hadn't been there before. Twisting his new wings around, he gasped despite himself. They were massive things. Red flesh with spines protruding at the joints. Opaque webbing stretched between the fingers and down to his back. Giving an experimental flap, he was tugged up a bit, not enough to achieve lift, but the power was certainly there. He also noticed the tattered bits of robe from his back. As the days went on, a robe was seeming less and less practical. He made a mental note of that, as he checked to make sure he had an extra for after the transformation.

After getting the feel for his new wings he walked over to the other slave and gently woke him with a kick to the stomach. To say the slave screamed when he saw Reaper would be an under statement. It's a good thing they were in wilderness and the only things startled awake were animals. Grabbing the slave by the back of his ragged shirt, he dragged him to the middle of the camp. "Stop squirming or it will be the worse for you," He said in frustration. The man stopped. "Your companion is dead. I killed him, but you are not next, understand this. I'm going to leave now and you're not going to run, do you know why?" The slave shook his trembling head. "You're not going to leave because I'm going to cast a spell on you and the camp as incentive. Should you try to leave after this spell is cast, you will be tortured to death and then your soul will be forced to go to hell. Not very pleasant, I'm sure you'll agree. The slave merely continued to tremble. Sighing, Reaper walked to the edge of the camp with his staff in hand. Remembering his knife, he went back to it, cleaned it, and placed it back in its carrier. Then he grabbed the carrier and strapped it to his back between his wings where it would be safe. Going back to the edge of camp, he picked his staff back up again. The slave was where he was before, hyperventilating. "You'd better not die of fright. I'd just bring you back and torture you for inconveniencing me. Now raise your hand in front of you palm facing the ground. Very good, now hold still." He didn't need as much focus for this particular spell. It was 1 part power 9 parts imagination. Light and a bit of heat, that's all. Cracking his neck and stretching his arms, carefully avoiding his wings, he outwardly prepared for his spell. None of it was needed of course. Most of this spell was for show. He used the base of his staff to carve a pentagram with demonic symbols at each point (drawn slightly wrong of course. wouldn't want to bring up any unexpected nasties.). Then he started chanting some gibberish words and thrust his staff into the earth before him. When his staff made contact with the ground, what appeared to be and pillar of fire shot from the pentagram. It was, in reality, just light, but it would serve it's purpose. When the pillar got about 50 feet above the ground, it formed into a massive dragon made entirely of flame. It then dived at the camp, flying once around it in a circle. The second time it circled the camp, it breathed more fire onto the grass, this time it was actual fire and it scorched a black circle around the perimeter of the camp. The dragon then flew high up and moved so that it was above the slave. Compacting its form slightly, it dove toward the slave's outstretched hand. Shrinking the whole time as it plunged, it was eventually hardly as wide as the slave's hand, but very long. Again adding a bit of heat, he plunged the dragon into its target. The slave screamed as the shape of a dragon was branded into the back of his hand. "Done and done" Reaper said to himself and his master, smiling at his handiwork. Picking up his small satchel of food, he flapped his wings and flew to the top of the mountain.