Never cast spells.
Don't be a magic-user or a wizard or a mage or a sorcerer. Spellcasters derive their power from the insane, transdimensional consciousness that hang bloated in the night sky over Earth like bricks with no concept of gravity. These god-things have no concept of what humans are, nor would they care if they did. Drawing power from such a source results in insanity, mutation, and Doom. The unavoidable kind.
They're doomed. And they know it.
Every miscast spell moves them closer and closer to an inevitable Doom. Basically, every time a spell is cast, there is a chance that it goes horribly wrong. And whenever it goes horribly wrong, the sorcerer gains 1d4 Doom Points. At 10, 20, and 30 Doom Points, something terrible happens.
10 Doom Points - The Doom of Fools
20 Doom Points - The Doom of Kings
30 Doom Points - The Ultimate Doom
The Ultimate Doom is aptly named. It marks the end of your mage's career, and time to roll up a new character. There might be ways to avoid or postpone your personal Ultimate Doom. And indeed, the existence of ultra-wizards (like Tesla) is proof that this is not impossible. Just the same, there are no established methods, both in Eldritch America nor in the rule books. If a Cure for Doom exists at all, it will be the result of a long and difficult quest. And as every aspiring mage who has ever picked up a spellbook knows, they will one day have to face their Ultimate Doom. That is, if the mutations, insanity, or lesser Dooms get them first.
Dooms are personal. Every mage invokes a different set of Dooms. However, these are the most common ones, based on what school of magic the mage practices. Some schools have it a lot worse than others. (The Cthonomancers actually benefit from their Dooms, oddly enough). None of them give any saves. THESE ARE DOOMS! YOU CAN'T SAVE VS DOOM! DO YOU SEE SAVE VS DOOM WRITTEN ANYWHERE ON YOUR CHARACTER SHEET? WELL DO YA?
Doom of Fools: Gain a minor mutation.
Doom of Kings: Gain a major mutation.
Ultimate Doom: Turn into a 1 ton mindless blob that eats everything and continuously spawns creatures from your shattered subconscious.
Doom of Fools: 1d3+1 Moon Beasts (3 HD) immediately appear via gateway and attempt to drag you back to the moon with them.
Doom of Kings: As above, except 1d4+2 Moon Beasts of (5 HD).
Ultimate Doom: As above, except 1d20+20 tentacles (5 HD) of Yosganeth himself emerge from the walls and ceilings within 300' to drag you back to the moon.
Doom of Fools: +1 Strength.
Doom of Kings: +1 Constitution.
Ultimate Doom: You
turn into a crystalline creature. Your
head resembles an inverted, featureless geode and your body is semi-translucent
and pale blue. You can no longer talk,
but get +2 vs poison and disease. You
are a mineral. You no longer have a
biology. You must make a save every month or you will finally accept that all your labors thus far have been in vain, and journey beneath Australia to join Cascrysmagog in sympathetic communion.
Doom of Fools: You erupt into a living inferno or ice storm (whichever is more appropriate). You fly around, shooting flames while being on fire and blowing shit up. Your only goal is the destruction of people and property, and you go about this task with insane, cackling gusto. Lasts 2d6 minutes.
Doom of Kings: As above, but lasts 2d6 days.
Ultimate Doom: As above, but permanent.
Doom of Fools: You have some sort of psychic seizure and shoot a lightning bolts out of your eyes in random directions every round for 2d6 rounds. There is a 40% chance per item that your gear is destroyed. Any small metal objects that you are carrying become fused to your body. (See also: Tetsuo: The Iron Man).
Doom of Kings: As above, except 50% and all medium sized metal objects carried become a party of your body.
Ultimate Doom: As above, except 60% and you automatically fuse with the nearest big metal thing. This could be a WWI tank, a septic tank, or a telephone pole. Attempting to intentionally induce this Doom in a controlled environment so as to fuse your body with something cool has a 40% chance of going horribly wrong unless rubber gloves, tesla coils, and goggles are somehow involved (which brings the chance of things going horribly wrong down to 25%).
Doom of Fools: Lose all spell-casting abilities for 2d6 hours.
Doom of Kings: As above, except 2d6 days.
Ultimate Doom: As above, except permanent.
Doom of Fools: -1 Strength.
Doom of Kings: -1 Constitution.
Ultimate Doom: You die.
Doom of Fools: You gain 1d4 insanity points.
Doom of Kings: You gain an insanity.
Ultimate Doom: Your body becomes the vessel for twisted alien intelligence, and your soul is shunted out into the cold void between stars.
The nice thing about imminent, unavoidable Doom is that it gives a strong mechanical thing for me to point at and say, "Don't be a mage. Mages are fucked." This sort of fits in with a Carcosa-style ethos.
At the same time, it's not entirely unavoidable. Electromancers have a built in way to engineer their Ultimate Doom. Cthonomancers don't have an Ultimate Doom that is as bad as the others. Necromancer's can avoid their Ultimate Doom by being a Lich. Psychomancers might sidestep their Ultimate Doom by abandoning their bodies early. This puts the initiative back in the hands of the PCs. They've seen their Doom approaching for a long time now, and they'll try very hard to find a Cure for Doom in order to avoid rolling a new character. Of course, the roads in Eldritch America are (almost) literally paved with the bones of mages who thought they could escape their Ultimate Doom, so success isn't guaranteed by any means.
Can you imagine playing with a PC mage who has 27 Doom Points? The next spell he casts could be his last before tentacles tear through the paper-thin walls of our reality and drag him screaming back to nameless places with impossible geometries. He's made some progress at avoiding the Ultimate Doom, but it's a difficult, frustrating process. He's considering abandoning it, and resigning himself to his fate. He could just avoid casting spells ever again, but that hardly seems preferable to suicide. He's a mage. Mages are doomed.