1. Raw Power:

Mastery might be based on the ability to control and channel raw energy efficiently.

Progress could be measured by the amount of energy harnessed or the scale of destruction one can cause.

Mastery could also involve developing techniques to focus and direct this power.

2. Shadow Arts: Assassins/People in this world can harness the power of shadows. The more skilled they are, the better they can manipulate shadows for various purposes like stealth, teleportation, or creating shadow constructs. The effectiveness of their abilities would be directly tied to their mastery level.

Marked Souls: Assassins possess a unique mark or symbol on their body that grants them extraordinary abilities. The more experienced they become, the more they can unlock from this mark, and the power of the mark could be linked to their inner strength or determination.

Domain: 

Domain Ability is a unique and potent power that allows its user to exert control and influence over a specific area or aspect of their environment. It often reflects the user's mastery and dominance in their chosen domain.

3. Weapon Mastery: Assassins gain power through their mastery of weapons. Each weapon they wield has a unique, supernatural ability, and the more skilled they are with that weapon, the more potent its abilities become.

4. Contract Magic: Assassins make contracts with mystical beings or entities for power. The terms of these contracts could vary, and the strength of their abilities depends on the terms and the strength of their connection with their contract entity.

1. Raw Power:

Mastery might be based on the ability to control and channel raw energy efficiently.

Progress could be measured by the amount of energy harnessed or the scale of destruction one can cause.

Mastery could also involve developing techniques to focus and direct this power.

2. Shadow Arts (Shadow Manipulation):

Mastery might involve the precision and complexity of shadow manipulation.

Progress could be measured by the number of shadow-based techniques an assassin can perform.

Mastery could also relate to the ability to blend seamlessly with shadows, becoming nearly invisible.

3. Weapon Mastery:

Mastery could be based on the proficiency and versatility with specific weapons.

Progress might be linked to learning new techniques or unlocking the weapon's hidden potential.

Mastery may also involve developing the ability to enhance a weapon's power through a deep connection.

4. Contract Magic:

Mastery may depend on the strength of the contract and the level of favor from the entity.

Progress could be tied to the complexity and potency of spells or abilities granted by the contract.

Achieving mastery involve negotiating better terms with the contract entity.

Leveling

1. Shadow Skill Levels:

Novice: Basic proficiency in the chosen shadow skill, with limited control and understanding.

Apprentice: Improved control and the ability to use the skill more effectively.

Intermediatre: Advanced mastery, allowing for more complex applications and greater precision.

Master: Near perfection in the chosen skill, capable of utilizing it in various creative ways.

Awakening Phase: A transformational moment where the character experiences a significant boost in their shadow skill, gaining access to a new, advanced technique or the ability to push their existing skill to new limits. This could be a pivotal moment in their character development.

2. Raw Skill Levels:

Novice: Basic proficiency in the raw skill, demonstrating some level of talent.

Apprentice: Improved mastery with more refined control and strength.

Adept: Advanced skills, demonstrating exceptional physical or elemental prowess.

Master: Virtuosity in the raw skill, often surpassing the capabilities of regular individuals.

Awakening Phase: A moment where the character experiences a significant boost in their raw skill, gaining access to a new, extraordinary ability or a substantial increase in their existing power. This awakening could be a turning point in their journey.

3. Spirit Contract Levels:

Initiate: The character has just formed a contract, and the spirit's influence is minimal.

Attuned: A deeper connection is established, granting access to basic spirit abilities.

Harmonized: The character and spirit have grown closer, enabling more potent and refined abilities.

Symbiotic: The character and spirit function as one, with an extensive range of abilities and a seamless partnership.

Awakening Phase: A pivotal moment where the character and spirit achieve an extraordinary level of synchronization, unlocking a previously untapped reservoir of power or granting access to unique, world-altering abilities. This awakening could redefine their role in the story.

Dim Aura: At the lowest level, individuals have a faint and barely noticeable aura. Their raw abilities are weak and not easily harnessed or controlled.

Glowing Aura: As characters progress, their aura becomes more visible and emits a gentle glow. This indicates improved control and a moderate level of raw power.

Radiant Aura: Those with radiant auras have a strong and vibrant aura that is easily visible even in daylight. Their raw abilities are formidable, and they can achieve impressive feats with them.

Luminous Aura: Luminous auras are exceptionally bright and radiant. Individuals at this level possess immense raw power and can perform extraordinary acts, such as manipulating elements or exerting tremendous physical strength.

Awakening Aura: The awakening phase marks a significant transformation in the aura. It could become blindingly intense or change to a different color, signifying the character's ascension to a new level of raw power. In this phase, they gain access to new, world-altering abilities tied to their raw strength.