While mainly seraphim, and to an extend malakhim, take on oaths to acquire powers they do not usually have access to, humans are also known to make them. Depending on what was chosen by the oathtaker to sacrifice for their newfound powers, failing to stay true to it may come with dire consequences. Melchior Mayvin took an oath to live for centuries, while Claudin Asgard took an oath that he will not kill in exchange for a long life. Seres took an oath that, in exchange for her life, a power would replace her, resulting in the formation of the Sorcerer's Ring after Velvet Crowe devours her.
Magilou took a special oath and became the Storyteller of her time period since she started to care and explore the world, resulting in her outliving other humans. After Maotelus fell prey to Heldalf, resulting in the loss of Silver Flame that can quell malevolence, Lailah took on the oath to not speak about the events leading to Maotelus's downfall at the hand of the Lord of Calamity at the time of Tales of Zestiria, in exchange for the power over Silver Flame. Symonne has taken an oath that grants her power over illusions that Sorey and his group speculate was obtained at the cost of vowing not to kill anyone, considering the fight she puts up in the depths of the Mt. Mabinogio Ruins.