Magic is a tricky thing. It doesn't always behave in a logical way and doesn't always get the results you wanted, which is why though magic is plentiful in the magic lands, professional wizards and magic-users are rare. For someone not used to magic, it can get quite overwhelming, and sometimes such a person might become irrevocably and permanently altered, in body and/or mind, by exposure to magic. This is known as being Spellstruck, and a person who has gone through such and event is referred to as "a Spellstruck."
(Just to be under an enchantment or be temporarily changed isn't enough, to count as Spellstruck the change must be permanent and impossible to reverse.)
There are several ways of becoming Spellstruck, but it always involves powerful magic. Someone who crosses over from the mundane lands to the magic lands have a high risk of becoming Spellstruck, as this involves getting exposed to magic for the first time. In this case the results and changes can be unpredictable; such a person might find themselves turned into a vampire, or having become a child again. Other ways of becoming Spellstruck is if a wizard or magic-user makes a particularly big mistake with a spell; there might be a magical backlash that changes either the wizard or people nearby; again the change might be random but is usually tied into what exactly the wizard was trying to accomplish. A third way is if a magic-user intentionally forces the change in someone, but this is considered highly unethical and is against all laws.
There are several degrees of being Spellstruck; sometimes the change is fairly minor and doesn't alter too much about the person. Sometimes it's purely physical, sometimes it's purely mental.
The most common form of Spellstruck, however, and by far the one people most fear to become, is a total physical and mental alteration. Such Spellstrucks have essentially become someone else entirely; they no longer look, behave or even think like the person they once were. In many instances they don't even remember ever being different. Some people claim that this is the true way to kill someone by magic -- while the victim does not actually die, the person they once were is gone.
Some people are more prone to getting Spellstruck than others. It's generally agreed that humans are the most susceptible, but it might easily happen to dwarfs and wights as well -- elves have a natural resistance, and dragons, who are already magical shapechangers, are almost completely immune.
Across the Five Known People, Hotbloods are at the greatest risk; they're not children anymore but they're not yet adults either. It's a time of great change and often great emotion, and both minds and bodies are more easily affected. In the very rare cases that an elf or a dragon became Spellstruck, it was always a Hotblood.
Children, by contrast, have little to worry about here; while children are very easily affected by magic, they also recover from it very easily. it's incredibly hard to get a permanent magical effect on a child; the magic usually wears off or is easily removed, and the child is none the worse for wear. In most cases where a child did become Spellstruck, the child was usually older and closer to the Hotblood years.
This might be why children of Spellstrucks very rarely become Spellstruck themselves. A child born to a Spellstruck will not inherit the parent's condition, and will most likely have developed a resistance that lasts into the Hotblood years and beyond.
In some places, "Spellstruck" can be used as a slur or an insult, usually towards someone especially foolish or irrational, but this is generally considered rather tasteless.