Tmesis (Greek, "a cutting") is a figure of speech in which a word or phrase is inserted into another word, often for humorous effect.
Tmesian Magic is a small school that deals almost entirely with scrolls and spellbooks. It is the art of booby-trapping spells by writting a second spell into the body of a larger spell. While this practice is uncommon with scrolls, as those have a tendancy to be sold or bartered, it is much more common in the case of spellbooks. This might seem to be an odd thing, since there are plenty of spells to ward said books, and even spells that in themselves are traps, such as spells that when read are activated, often exploding on the reader of the book.
Would an ancient culture really leave such virulent traps for those centuries later to find? I would guess probably not. A more pressing concern is going to be their peers and rivals mages. What mage has not thought of purloining another of his peer’s spellbooks for access to his spells and spell research? There would be an unwritten but understood code of honor to see spellbooks returned to their owners should they be lost or stolen, entrusted to their next of kin. The threat of finding one of these spells carefully placed into a spellbook could be enough of a deterent to keep some of the slighty unscrupulous from abusing or purchasing a stolen grimoire. For those who would not even think twice, their fate is a bit more messy.
The trick behind the Tmesis style of scribery is that the writting mage knows the section of the spell that is the tmesis element, and can invoke the spell without using that portion. Those the mage instructs in the style are also able to use the texts without undo harm. Some mages could be consistant, with Emberwyld the pyromancer making the third verse of every spell incantation his tmesian trap. The mage could also change things around, just to keep a canny apprentice or magic reading thief from figuring out his system.
Generally, not every spell in a grimoire is going to be so booby-trapped, and in all likelyhoods, fewer than 10% of the spells present are going to be treated so. Doing more than this takes up valuable room in the text, as well as time writting the spell. Also, the spell being booby-trapped must be larger than the trap itself, otherwise the tmesian element becomes rather obvious. First or very low level spells are not going to be trapped, while mid to high level spells are much more likely to be. And as a rule, the effect of the tmesis aspect can mimic a spell of two ranks lower, or alter the way the spell itself functions.
A canny mage can attempt to check a spellbook for such spells, but first the mage must actually know of the tmesian school first, and must be familiar with the spell being cast in question. For example: Baldur the Unwise has gained a stolen spellbook, and has been afflicted with the Nocuon tmesis. He reads carefully through the book, checking the spells he is familiar. After checking each spell (A spellcraft, magery, or similar mundane skill) he decides that the two fire spells are safe, since he knows both of them. A third fire spell has a tmesis in it, an dhe will avoid it as he doesnt recognize what sort of tmesis it is. The three other spells are a different matter, two are from a school he is not very familiar with, so he cannot be sure if the spells are safe or not.
Thus presented for the approval the citadel, the Scroll of Tmesian Magic.
Additional Ideas (6)
The tautologos is a very devious trap found in higher level spells. When the caster of this spell invokes the arcane forms and energies his spell is cast without incident. However the next time the mage attempts to cast a spell, even if it is a cantrip, the tautologos spell will activate, and he will try to cast the same high level spell again.
Thus Barnabus the Barmy casts a powerful Summon Planar Demon from the pilfered spellbook of an evil cultist, makes a deal with the demon and then is on his merry way. Two days later he decides to cast a Charm Person spell on a comely young tavern lass, and instead invokes the Three headed Lord of the Fifth Pit, He Who Revels in Disemboweled Foes. Mayhem ensues.
Of course, if the mage doesnt have the spell components to cast the spell, he looses XP equal to the GP value of the spell components, if such are used. If the mage doesnt have the ability to cast a spell of this magnitude again, he takes one large die of damage per difference in spell level. In the above mentioned example, assuming the demon summoning a 6th magnitude power and Barnabus only has a slot left to cast a 3rd magnitude power spell, he is going to take 3 dice of Strain damage from trying to cast beyond his means.
Tautology - needless repetition of an idea, statement, or word
This is also a fairly insidious tmesian trap spell. Most often placed in dangerous summoning spells, potent scrying spells and elementally aspected spells. This is a smaller spell that can be placed in mid level spells without being noticed. It's effect is fairly simple, as it inverses some aspect of the spell it entraps.
For example, Tamur the Great has stolen a spellbook from Angia the Summoner and decides he is going to use the previously mentioned spell, Summon Planar Demon. This spell has an evil/unholy descriptor and he is ready to start wheeling and dealing with a demon of death and decapitation. Much to his surprise, he ends up summoning Radia, Angelic Defender of Children and Light. Seeing the vile and contemptable owuld be diabolist, the confrontation is sudden and violent.
Other less dramatic examples include altering fire spells to ice spells, turning poison cloud spells into happy cloud spells, and curses into boons.
Enantiodromia - the changing of something into its opposite
This is the most straightforward and potentially lethal of all of the tmesian spells. All spells involve the concentration of magical energies in a safe manner. The Nocuon actually draws in these magics in a very hazardous manner, channeling them through the vulnerable flesh and bones of the caster causing magical burns. The more powerful the spell, the more signifigant the damage inflicted.
Going back to the 6th magnitude Summon Planar Demon, Luchilde the Insolent decides to cast the spell from a stolen book of a slain rival. He performs the various gestures as dictated by the book and as he invokes the spell after several minutes of chanting he is filled with immeasurable pain. AS the spell is of the 6th magnitude, Luchilde the Insolent takes 6 dice of damage, and with a mage, often weak in health/hit points, this could very well kill him on the spot before the spell is completely cast.
nocuous - harmful
Render unethical mages blind, deaf, and dumb.
Cataractia - the spell causes the mage in question to become magically blinded.
Tinnitism - the spell causes the mage's ears to be filled with a constant ringing. While the sound is eventually adjusted to, the mage is left effectively deaf.
Dysphonion - The last word of the spell robs the mage of the ability to speak, rendering him mute.
Less sensitive material might be guarded with more innocuous magics woven into the text: Casting causes a wizardly mark to be placed on the caster, but visible only to those of the correct magical tradition: a warning that he cannot be trusted. Such would normally be used to identify disobedient apprentices that have experimented with material they are not yet approved to study.
graphein - to write
The spell includes another magic that is as nearly opposite in effect as possible or otherwise tends to cancel it out. For example, an illusion spell might be linked to a spell generating a cloud of fog.
alexein - ward off
The secondary magic is one that knocks the caster unconscious the third or fourth time that the main spell is cast. Because it takes several castings for the secondary effect to manifest, casters are likely to conclude that the spell is safe for use and then be caught by surprise.
amnesia - oblivion
Incurion is a particularly devious tmiesis, in that its effects, by definition, go completely unnoticed by the individual- very often, the object is one not found in the spell the tmiesis is hidden in; rather, the feeling that something isn't right will only begin to dawn on the caster slowly. Spells will fail or misfire; the enchanter will get odd looks in conversation as he skips syllables in his speech- minor annoyances at first, but enough to unnerve or madden the victim.
Must be removed by a second party, as the victim, even if somehow aware of the enchantment's effect, is completely incapable of determining what, precisely, the object is, and what words it's found in. Easily detected by asking the victim to say a word containing the object- useful in the case of wayward apprentices.
incuria - carelessness