Experience Debt, or XP Debt. Is a mechanic that slows a character's ability to gain experience points for creating and improving Metaphors or obtaining Attribute Training Points (ATP). Characters can receive debt in a number of ways including balancing for powerful characters, as well as penalties due to in-game events.
Debt Mechanic
XP Debt is a value the player-character must track before adding any experience points to their sheet. They can sacrifice any amount of gained experience in any amount to clear the debt faster. Each instance of debt should be noted and tracked, so if a character has 10 points of Debt, remember 10.
Whenever a character gains experience, they must first choose how much debt they want to pay off. Only by having paid at least 1 point can they choose to bank an equal amount for normal uses of experience. Once the debt is paid in full, the character can gain experience normally. If a character only gains a single point of experience, it automatically goes to paying off the XP debt first.
"Zanku, a playable wish-granting Djinni, has 100 points of experience debt (due to being a powerful extraplanar with Wish-granting abilities). When they gain 3 XP from some event, they had to pay off 2 points of debt before banking the remaining 1. Later they gained 4 XP, and paid off 2 points of debt to keep 2 XP for themselves. The one time they got a single point of experience, it was spent automatically to pay off the debt. After enough play to pay off all 100 points of debt, Zanku was free to gain experience points normally for all other rewards!"
Uses of Debt
As a Game-Master, you may want to impose XP Debt onto player-characters as either a balancing approach, or as a penalty due to actions or events that took place.
As Balance
You may decide that, at a player's request, you allow them to play as, or use Metaphors/powers that go beyond the baseline for player characters for a game. Things such as having magic or powers when characters shouldn't have them, allowing them to play as a Giant or Monster with abilities exceeding that of normal humanoids. When a Game-master allows a player-character to be much stronger than normal, they can set an amount of XP debt in exchange.
A reason to do this is to allow the powerful character to retain the power they should, but give the others an edge when it comes to improving their characters at an accelerate speed compared to the one who received their power early. It limits the strong character's progress so that the others may eventually catch up.
"You run a fantasy game, and one of the players says they want to play as a Rune Giant adventurer, this Rune giant is likely to be about Size 8 and also have a bunch of rune-based powers based on their species. Despite the immense gap in power compared to a regular humanoid, you allow this, but state that this Rune Giant character of theirs has a Debt of 70 due to their species. When their character has paid off their debt, they can start earning experience like a normal character."
As Penalty
Occasionally, a character might get into serious trouble, or even get cursed or killed. While the Status magnitude mechanic can definitely fill most gaps regarding injuries and ailments, one option is to have a character suffer XP debt instead. A character could suffer a traumatic injury that needs time to heal, physically and mentally, and this injury could affect their ability to gain experience rather than outright debuff their capabilities or powers.
"A character was captured by horrific ghosts and spectres, while they were saved in time by allies, the trauma persisted. The Game-master asks the character to roll a Charisma check against a magnitude 20 Experience-Debt condition. The player character rolls a d12 and a md6 (for +2 Charisma) and got a 7 and a 2. With a total of 9, the GM rules that the character has 11 points of XP debt. When the character finally pays the debt, they come to terms with what happened and are able to move on."
XP Debt Guidelines
Here are some example values you can use to justify XP Debt.
Extra Metaphor
"Player requests an extra Metaphor for their character beyond the ones they get for free at creation."
Cost: 5 Debt per Metaphor created at +1, add 10 Debt for value above +1 for that extra Metaphor (+2 costs 15, +3 costs 25...etc).
Extra Trait
"Player requests a functional trait that enhances/modifies their character."
Cost: Determine how effective this trait is and assign debt value between 1 and 50.
Increased Size
"Player requests to enlarge their character to a size bigger than their species allows."
Cost: 25 Debt for each Size rating bigger than 1. Lower to 10 Debt if the character's species is Size 2 or bigger normally.
Raise Base HP
"Player wants to have higher base HP (HP die), such as going from a base-6 to a base-8 (starting HP 8 + 4 per Body/Lv)"
Cost: 10 Debt for each increase to base HP, up to the GM's limit.
Gain Superior Qualities in Attributes
"Player wants to improve certain attributes to gain Auto-die and/or Level-Boost qualities and their respective +1s."
Cost: 15 Debt for each quality added to an attribute (30 for both on one attribute).
Revival/Survival
"Player's character has 'died' but they still want them to still live"
Cost: 10 Debt to retcon character's death into something less deadly. Increase cost by 10 each time this occurs.
Drained
"Player character suffered significant loss to their powers and/or ability, like being drained or enervated."
Cost: Determine how severe the affliction is and assign a value between 15 and 30. Player may roll to reduce Debt further once at the beginning.