I'm trying to quantify the use / restrictions with the Seeming for my 5e campaign. I think I have it balanced for 5e, happy to take discussion on this.

Seeming Score
Rating Score Modifier Effects (as a full round action) Duration
Slight 1-5 +0 Tiny, temporary, obvious illusions 1-5 Seconds
Lesser 6-10 +1 Small sized illusions, semi-solid objects 1-5 Rounds
Moderate 11-20 +2 Larger, more solid 1-10 Minutes
Greater 21-40 +3 Creations are solid and almost permanent 1-20 Years
Extraordinary 41+ +4 Can create their own realm and define local reality Permanent
Seeming is the raw material behind illusionary, divination, and necromantic magic. However, it usually goes undetected unless the observer has some understanding of Seeming. The Seeming score refers to how much exposure a person has to Seeming, or how much they are a part of it. For instance, a scholar from the Daylight World might spend a year studying books on the topic before gaining their first point of Seeming score. In contrast, a young person whose nightmare is so vivid it draws the attention of a Shadow denizen might wake up with the same level of knowledge, even if it is only in their subconscious. This knowledge does more than just grant understanding; it starts to shape the individual, since the knowledge of Seeming and the Seeming itself are intertwined. Residents of the Shadow World typically have a slight to moderate rating, with a matching score between 1 and 20. In comparison, powerful denizens of the Shadow World and those who govern their own realms will have a score greater than 20.
The score is used in two ways. When viewing an effect or illusion created with Seeming, if the viewer’s Seeming score is higher than the caster’s, then they are aware ‘something is not right’ and have advantage to perceive the reality. If the viewer exceeds the caster’s score by more than 10, then they automatically succeed. Secondarily, a conjured Seeming effect lasts a maximum of as many time intervals as points of Score within their Rating bracket. For example, a creature with a Seeming score of 7 could create a playing card that would last up to 2 rounds (12 seconds).
When making a Seeming Perception test, roll d20 plus Seeming modifier plus Intelligence modifier. Note this does not include proficiency bonus as proficiency with Seeming is about exposure, not general experience.
Effects are rated based on their intensity, scale, and magical complexity. Here are examples for each category:
• Slight: The most subtle effects, noticeable but fleeting or limited in scope. Examples include:
o Object: a small, two-dimensional apple
o Sensation: a faint scent of roses in the air
o Spell: a gentle 5’ moonlight glow
• Lesser: Mildly impactful effects, capable of minor manipulation or sensory alteration. Examples include:
o Object: an ordinary playing card
o Sensation: the feeling of cold water on your skin
o Spell: a Minor Illusion
• Moderate: Noticeable and tangible effects, often comparable to basic magical spells or objects. Examples include:
o Object: a human-sized ice sculpture
o Sensation: a sharp crack as if from a whip
o Spell: moonlight filling a 100’ radius or the effect of Chill Touch (Necromancy cantrip), or level 1 illusion equivalents.
• Greater: Powerful, complex, or large-scale effects that may alter the environment or influence many at once. Examples include:
o Object: a suit of armour or a magical weapon
o Sensation: a sudden chill that affects everyone in a room
o Spell: equivalents of level 1 divination or necromantic spells, level 2 illusions, or subtle shifts in the landscape (likely to attract the attention of the realm’s ruler)
Restrictions
The effects listed above only work in the Shadow World, or at the site of breaches in the veil.
Entry into the Daylight world becomes more difficult. When attempting to use a Shadow gate (or assisted by a halfling), a character must make an Intelligence saving throw against a DC equal to their own Seeming score to enable entry. If the save fails, the character cannot enter and must try again later.
Staying within the Daylight World is also more challenging for those strongly tied to the Seeming. Each dawn, regardless of where in the Daylight World the character is, they must make an Intelligence saving throw with their Seeming score as the DC. On a failure, one of two things will happen (players choice):
o They are either pulled back into the Shadow World (meaning they immediately vanish from the Daylight World and reappear in the Shadow World, unable to return until conditions allow another attempt)
o The character takes one level of exhaustion for every ten points they failed the save by (rounded up). Exhaustion levels do stack, so repeated failures will accumulate, potentially leading to severe consequences as per standard exhaustion rules (e.g., reduced movement, disadvantage on checks, and so on).
For example: If a character has a Seeming score of 15 and rolls a 10 on their Intelligence save at dawn, they fail by 5 points. Since the failure is less than ten, they would take one level of exhaustion. If they rolled a 4, failing by 11 points, they would take two levels of exhaustion. If they failed by 20 points, they'd take three levels of exhaustion, which would stack with any existing exhaustion.