Bay 12 Games Forum

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  ■ It curbs the ubiquity of magic in the world. The more powerful the spell cast, the more time you lose from your max age. The weakest cantrip could cost minutes or hours, a very strong spell could wipe off a decade. Truly awesome magic might kill you instantly. Or it could be random for all magic (but too much random cost would limit plot hooks, I think).
  
  ■ If you don't have enough years left to live you can be stopped from casting the spell, you could cast the spell and die, or you can just die without casting.
  
  ■ Immortal creatures. Options include locking them out completely from age-burning magic. Losing their immortality if casting spells and then aging like mortal creatures with a max age. Having a certain number of years they "use up" until they can't work magic anymore while they wait for their years to "refill" (mana bar). Or immortality could just grant unlimited age-burning magic, potentially making immortal creatures powerful magic users relative to mortals.
  
  ■ Access to certain spells could depend on race (and/or lots of other things). Lets say that immortal creatures don't lose their immortality and can cast as much as they want. That doesn't necessarily make an elf super powerful compared to mortals if all the elf can cast is "raise garden bed" compared to the mortals library of death-dealing magic. In other words, being a creature with a greater max age doesn't necessarily mean you are automatically a stronger magic user compared to a shorter lived creature because it could still depend on which spells you have access to. Having said that, many of the immortal or long-lived creatures are often supposed to be more magically powerful, like dragons etc.
  
  ■ Using magic could visibly age you prematurely in line with the amount of lifespan you're sacrificing. A gratuitous magic user of 30 could have the decrepit body of a centenarian. This could work as a tell to let you know when to ease up on the magic missiles ("You look like an irresponsible magic user indeed." "I'm six!"). Sacrificing physical strength/health in order to gain magic power is an established trope. Or instead of visible aging and the accompanying physical weakness that entails, you might change in a more eldritch fashion. You know, weird eyes, scaly skin, growing extra or monstrous appendages which could make you more physically powerful during your shortened life, or subject to prejudice or a myriad other consequences. Or maybe there's no indication at all as to how much of your lifespan you've burned up, and you look and feel exactly your current age, despite the fact that you will croak in 5 minutes from old age despite being 15.
  
  ■ Magic that relies on lifespan naturally lends itself to individuals trying to cheat their way around it. So evil beings might search for/discover a way to use magic by sacrificing another creature's years rather than their own. It could be a 1:1 ratio, or less efficient. A less efficient ratio, like using up someones entire 50 years granting you free use of 5, could encourage industrial levels of sacrifice to fuel the spell casting of a tyrannical sorcerer-king or despotic theocracy etc. Other plot hooks include a civilizations rulers grooming a caste of citizens to act as disposable life batteries for use of the ruling class.
  
  ■ Instead of sacrificing someones else's years at time of casting, you may be able to leech another's life span to replace your own after the fact of casting. This could work as a world's version of vampires, or in addition to them.
  
  ■ It doesn't all have to be evil sacrifice. You're heroic and noble companion might sacrifice their years to empower you to defeat the demon you couldn't otherwise challenge.
  
  ■ Magic item creation could also require life span sacrifice which would put downward pressure on the number of powerful magical items in a given world. It might also be a cost in dwarf fortress strange-mood artifacts (although that'd probably be unpopular with most players, I guess). Alternatively, or even additionally, using a magical item, or using it with it's magical properties in effect, would cost you lifespan in the same way as casting magic. There's huge scope for options in this, like using a magic sword being "free" as long as you're regularly killing things so that it saps their life force rather than your own. Or if you're using a sword in strict accordance with some embedded criteria (only killing goblins under moonlight while standing on one leg and whistling) you don't use up your life.
  
  ■ This magic system could lend itself to "Oh well, I'm gonna die anyway so I may as well press the Armageddon-spell button now" situations which may or may not be a bad thing. There are lots of ways to counter that, though. Armageddon-level spells requiring a very long casting time is an easy example, so you can't cast one because you've just been disemboweled and max life span is no longer important to you.
  
  ■ There's no reason this magic system can't sit alongside others in a world. Although if it isn't relatively powerful in comparison to less punishing means it'd likely be obsolete, which is not necessarily a bad thing.
  
  ■ How to train and gain mastery of this type of magic raises important questions. Do you have to spend some life span to learn or practise to cast the spells? The stronger the spells, the more life you've sacrificed learning them? That would mean that the most powerful casters are also the ones with the least time left to pursue their agendas. Or maybe the more practise done or expertise you've gained lessons the age cost? Or maybe the most desperate people are the ones who choose to delve deeply into this magic. Someone vowing revenge on an enemy they can't hope to defeat normally might decide its worth sacrificing their life to learn and use the magic to achieve the desired outcome. Lots of plot hooks could depend on how you learn and gain power in this type of magic. Of course, learning it could just be via full revelation and free with the only cost being the actual use, and that'd create a different world.
  
  ■ Problem: how does a randomized system, generating randomized magic, decide how powerful/utilitarian a spell is and thus give it a proper lifespan cost? Leaving aside balance during actual play, all sorts of world generation bugs could happen if important historical figures are dramatically shortening their lives in silly ways. I don't think it'd be insurmountable for Toady, but I don't think it'd be trivial either. I doubt it'd be a implementation problem unique to this system of magic though, if it even is a problem at all.
  

The long-term (as in, 20 years) plan is for almost nothing to be abstracted, so there's that.