Particularly powerful spells may semi-permanently weaken a caster, bumping them down a Power Level for years until they recover. RPGs, both cRPGS and tRPGs, have Experience Points as a ready-made resource for this kind of thing - and it's an especially useful trope there, since the game is likely to end long before the natural lifespan of the characters, meaning that from the players' perspective, Cast from Lifespan is consequence-free. XP, on the other hand, is something even a Munchkin cares about. note Another common variant is to have important spells cost stat points (which then can be restored with XP or training, as per usual). The extra power of the spell is usually justified to be because of the sacrifice involved, making the XP cost a Necessary Drawback.
This trope sees particular use with abilities which are important to have around for plot purposes, but whose indiscriminate use would render the setting unrecognizable in short order.
Many Dangerous Forbidden Techniques in the Xianxia genre are cast from XP, something which neatly explains why martial artists don't launch their most powerful techniques right off the bat, but only use them when absolutely necessary.
Subtrope of Power at a Price. Compare to sister tropes Cast from Lifespan, where the caster can't recover (and in many cases aren't noticeably weaker), Cast from Hit Points, where the caster is hurt by the spell, but recovers as from a normal wound, Insert Payment to Use, for artifacts that requires a price to be used, Cast from Money, which is magic that requires a monetary expenditure to work, and Cast from Sanity, which is when magic is harmful to your mental health. The default is for spells to be cast from Mana.
- In The Neverending Story, Bastian's wishing power turns out to require sacrificing memories of the human world.
- The eponymous protagonist of the Allie Beckstrom series by Devon Monk loses her memories when she performs magic. The greater the magic, the more memories she loses.
- In Harry Potter and the Methods of Rationality, several spells are powered by Sacrifice, including Fiendfyre, which permanently decreases your blood volume, and the Unbreakable Vow, which permanently decreases someone's Magic reserves (thus explaining why it's not routinely used to enforce business deals).
- Dungeons & Dragons
- In 3e crafting magical items costs XP, with powerful spells like "Wish" and "Miracle" also costing XP.
- In 3.5, the following spells require XP: Planar Ally (all spells), Atonement, Commune, Greater Restoration, Gate, Miracle, Awaken, Permanency, Vision, Simulacrum, Limited Wish, and Wish.
- The 1st/2nd Edition compatible supplement The Tome of Mighty Magic has a spell called Divine Retribution. If the spell fails, it drains a level of experience from the caster.
- The 1st Edition spell Energy Drain. There is a 5% chance the caster will be drained of a level of experience.
- Energy Drain returns in later editions as a debuff, in the form of negative levels. At first, energy drain inflicts a temporary negative level, sapping hit points and inflicting a 1:1 penalty to all rolls and statistics. If this level isn't removed, and the save against it is failed the following day, the negative level may become permanent, forcing the hapless character to earn their level back the hard way.
- In Numenera, you can use unspent XP for temporary advantages. Interestingly, you can also gain XP by accepting disadvantages.
- Vampire: The Requiem has a few of the most powerful Theban Sorcery spells cost Willpower Points; once cast, the stat itself goes down. Characters can spend XP to buy it back up to the previous level, but until then they're essentially less able to expend mental and emotional energy. Interestingly, creating a new vampire requires the exact same expenditure on part of the vampire's sire. The setting details some sires who mass embrace too many, too quickly, and become emotional wrecks unable to hold themselves together.
- Princess: The Hopeful: As with Vampire above, a few of a Princess's most powerful Charms (most notably crafting a Bequest or empowering someone as a Sworn) cost a dot of Willpower, which must be bought back with XP.
- In Exalted, Charms that give permanent upgrades to oneself or others tend to cost XP.
- Form Fixing Method, used to give a Lunar their awesome magical tattoos.
- Youth-Restoring Benison, used to restore a mortal to full youth, costs 1 XP if the effects are to be permanent.
- In Swedish RPG Trudvang Chronicles, the spell to make a permanent magic item costs 1 permanent Willpower, and the one to establish its own power supply (rather than drawing mana from the user) costs an additional one. On the bright side, Willpower increases get progressively more improbable, the more you already have (roll below 25-WP on a d20 after an adventure where you use your WP stat), which means that spending Willpower makes them easier to get.
- RuneQuest also requires permanent points of Power (the magic stat) to be spent when making permanent magic items, learning Divine Magic (the most powerful magic in the game), or when making a Fetch to become a Shaman. Like DoD, it's easier to (re)gain Power when the stat is low.
- The Projects System in Nobilis, which is the formalized way to measure your progress towards goals like "Freeing the Damned Souls in Hell" or "ousting Lord Entropy", is also the system you use to increase your own stats, meaning there's a more or less direct trade-off between having large-scale impact on the setting and getting an overpowered character. (Not that Nobilis characters aren't overpowered from character creation anyway...)
- Late in A Dance with Rogues, performing a specific, particularly selfless act (freeing the fallen paladin Luciano from an eternity of enslavement by a powerful demon to give him a chance at redemption) costs you one tenth of all of your accumulated XP, potentially bumping you down a level permanently.
- The old DOS game Wizard's Castle has the Web spell which prevents an adversary from launching an attack for between one and five turns, but subtracts one strength point per use. There is also the Fireball spell, which delivers anywhere from one to fifteen points of damage. Fireball subtracts one strength point and one intellect point. One cannot cast spells in this game with 14 points of intellect or less.
- In Minecraft, experience points and levels are spent when enchanting items, repairing items, or automatically consumed by the Mending enchantment.note It should be noted that experience points in Minecraft do not make you stronger as you accumulate them - you're meant to expend them in this manner.
- Some of the hexes in Dark Souls II use up souls to cast. One uses all souls the player has, but when the count is 5,000 or higher the spell's power reaches One-Hit Kill levels. Since souls are both XP and currency, it's Cast from Money at the same time.
- In Heroes of Might and Magic V: Tribes of the East, Zehir was granted by a Djinn the ability to summon fourth a flying town using a spell. It was only when he first summoned said town that he realizes the spell costs experience points to use instead of conventional mana (justifying Bag of Spilling when it comes to his levels).
- In Crimsonland, one of the perks you can choose lets you shoot your gun with an empty clip, at the cost of a hefty amounts of points for every shot. (Your score determines when you level up.)
- In El Sword, Lu was once a powerful demon, but she got sealed by other demons. In order to escape, she used most of her energy - and succeeded - but she turned into a little girl as a result.
- In Nocturne (RPG Maker), Reviel can focus all his might to destroy certain obstacles at the cost of experience points, allowing him to bypass certain puzzles. Given that the game has Low-Level Advantage and that experience points can be saved up before being spent on improving your level, you are likely to have more experience points than needed, making this example less severe than most others.
- In Before the Echo, you have to spend a certain amount of XP for Item Crafting, You can spend more XP to increase the chance of success (up to a maximum of 95%), but any XP points (and levels) you lose from crafting have to be earned again through battle.
- In Omega Labyrinth Life, the most powerful spell available is the Omega Slash, dealing massive damage to all enemies. Without using a consumable tome of the same name, it will cause your character's bust to shrink back to normal size and take the associated stat bonuses and special skills with it.
- The God of High School features a character who essentially does this, manifesting as aging in reverse when she uses her powers. Essentially she turns into a child, or could even de-age out of existence if she doesn't wait years between using her abilities.
- In Maou The Yuusha, an overlord creates servants from their own essence, decreasing their own stats in the process. Most Overlords send their servants out to level, then reabsorb them, gaining a net XP increase.
- In Against The Gods, Forbidden Techniques are fueled by Heart's Blood, something which typically causes the user's Cultivation Level to drop.