All forms of reading becomes inherently dangerous.
TL;DR - Readers develop a short form of pre-reading to "hash" the text and compare the hash to known enthralling patterns to detect traps. "Abandon your mind to the..." becomes "Abyo Mito Ta.." (nonsense words) which don't mean anything to the reader other than, "this is a trap"
- It cost no mana (no magic response)
- It cost few money
- Protect you even when you are not ready
- Allow you to read what you want, in every language you know
- Doesn't take time
Only costs a second or two when reading something to check the hash, and the education to learn and enforce the habit among the average reader.
Though this isn't the only way the world would react to this possibility.
This threat would be akin to mass mailing anthrax IRL
This would be a global security concern if stealing someone's mind and making a thrall out of them was really this simple, amassing slave armies every time you put up a new billboard, entire wars could be won by painting your words in the clouds above or using illusion magic to "pepper spray" an attacker with hovering symbols that a fast reader simply glancing at would fall prey to. To then have the concentration and effect sustained by the target itself thereafter, that's no simple task.
This must be either, much more difficult to perform than explained, or newly discovered and never used against the world yet.
A less severe version may be a book that contains a glyph which houses "Command" or "Suggestion" like effects to "READ" or to continue reading the control spell until finished, so that the hook has a definitive sink that can either be resisted or fallen for. From there, the duration of time to read the enthralling symbols is all that waits for the effect to stick long term, though the compulsion to read doesn't mean the target isn't aware of what's being read.
A commoner likely would just read and absorb the information but a wizard would likely be able to identify the intent of the trap setter by understanding the template of words being read. As in general safety training, specific dangers as these sorts of traps would be wise to teach in schools to prevent strong magic users falling prey to these simple traps.
I believe the response to such an attack would be immediate, urgent, and far spread when detected. Once discovered every magical defense program would be working to put an end to it and devise ways to find who set the trap.
This gives us 4 points of resistance
-Before you see the hook but are within threat range.
-The compulsion to continue reading once the hook is activated.
-The formation of the lasting enthrall effect while hooked.
-Post attack response.
Before you see the hook but are within threat range.
Important places such as government buildings etc will be outfitted with Magic Dispelling gateways. When walking through, all magical items much be handed to a guard/wizard to be checked via detect magic to determine any enchantment or illusion school magics (if detected further probing occurs), while everything else and the person passes through a dispel magic portal. This removes any magical effects present in items or creatures passing through (no disguise self allowed) and the magic within the text that creates the link between the trap setter and the target is removed, disabling the trap.
In general, an enchantment effect found using detect magic on either an object or person would become much more suspicious and actionable, towns likely setting up guard patrols specifically containing magic detectors to scan areas regularly as a result.
The compulsion to continue reading once the hook is detected.
Readers would be trained to read in a way that disrupts direct thought injection attacks. Rather than reading entire words, a learned habit to replace words, shorten words, or rearrange the words read can be used as a passive defense.
Rather than reading "Abandon your mind to the..." would instead be read as "Aba-yo mi-tota..." which will become a barrier allowing the reader to understand abstractions and concepts indirectly (imagine a code hash) and compare that abstract with commonly used "enthralling" patterns to determine that an attack was attempted before reading any further. Correspondence to high importance folk would filter through a scholar that could screen for traps.
Additionally, divination sensors can be set to Alarm whenever "I am magically compelled to do something" to emit concentration breaking sounds or to alert others nearby that they have been enchanted on repeat (imagine magic mouth necklaces in DnD5e).
The formation of the lasting enthrall effect while hooked.
Contingencies and emergency response could be pre-planned, "If I am ever magically compelled to do something, cast dispel magic/banishment/etc on myself and send a message to X for help."
Passive defenses must be developed to have better control over your own mana. From a suit, to a ring, or even a magical tattoo, this will prevent any unwanted mana escaping from the user. To cast spells, all mana is funneled through this "mana gate" as a part of the effort in casting the spell. Once the spell is complete, the gate closes. Specific spells that require an open gate can be configured to allow a constant stream separate from the casting of the spell required. Once compelled to link the thralls mana to the trap setter, the mana gate disallows this stray user mana being applied to the constant effect of the mind control, preventing it from being established or sustained.
Post attack response
Uncertain if thralls are aware they are being controlled and can be commanded at will, or if they become mindless drones to the controller (how to manage that many thralls at once without them appearing to act oddly) but we're going to assume a thrall cannot seek to break themselves free on their own.
As a response Warding Glyphs could be placed in high traffic areas, "If a someone being mind-controlled is detected, cast dispel magic on them" to create barriers of entry for any thralls sent to these location, or to prevent thralls from leaving if they fall prey to a trap brought inside.
Catching one of these thralls or traps would be the swift end to the trap setter though. The connection forming magic within the trap itself is the mana of the caster. The object can now be used to trace divination back to the trap setter. If a thrall is found, the mind control connection is maintained by the mana of the thrall, but the commands must be linked to the trap setter. Divination magic would likely be able to follow this trail as well, unless the magic was specific enough to release the captured thrall if such an attempt to trace was made.
If all it takes to get the spell setup is to inject a thought into someone's mind, you could use similar magics to sending/message/telepathy to simply inject these words directly into the target? If that wouldn't work, then similarly, I don't see how reading the text would be any more effective (other than in the methods described in Symbol or glyph ward that are triggered by being seen.
The problem I see is that after the impulse to read has been delivered, the trap setter stops being the caster. The caster becomes the thrall, who then self-casts a "Give my mental control to another target" spell on themselves, with the target they hand control over to being unknown to them.
In most examples I have, you can speak the verbal components of a spell without it's effects occurring if explaining them in conversation etc. You have to put more into the spell than simply saying words. I don't know how you'd force a reader to put that extra effort in to cast such a spell if they
- A) Understand that it's enslaving them (they would not put their own mana into the spell, but instead just read the words as compelled)
- B) Read the words and have no idea it's a spell (which again, no effort to exert their own mana while reading something they assume isn't a spell)