I just watched an episode called:
What is this さらわる? Can it be written with kanji? Any reason why online dictionaries don't seem to understand its structure?
First, you should know that conjugated verb forms will not be found in dictionaries, paper or on-line. All you find generally are the "dictionary" forms.
「さらわる」 is the literary (and classical) passive voice form of 「さらう」 ("to kidnap"). Its modern counterpart is 「さらわれる」.
The passive voice forms:
Classical: 「～～る」 and 「～～らる」
Modern: 「～～れる」 and 「～～られる」
The old form is still in use when the author sees it fit for his/her aesthetic purpose. It is used in titles quite often. To the native ears, the old form sounds more dramatic and just plain "nice" for a lack of word, but that is just like how older forms are treated in English, isn't it?
So, 「～～さらわる」 means "~~ kidnapped" to use the headline grammar.
In prose, of course, it would be "~~ gets kidnapped."
To write さらわる with kanji: 攫わる (this is for the "kidnap" meaning of さらう)
In Classical Japanese this was the verb さらふ which is conjugated with ハ行四段活用, like 言ふ.
To make the passive form of さらふ you need to add the Classical Japanese 助動詞「る」 which attaches to the 未然形 (this is 「は」 for ハ行四段活用)
This is how you get the meaning of, "to be kidnapped".
There are other, somewhat similarly constructed words which are still in common use as verbs on their own in Modern Japanese, which you can look up in a Modern Japanese dictionary:
さらわる is constructed from similar rules, but is not recognized as a verb in its own right for Modern Japanese, so you wouldn't find it in a modern dictionary as「さらわる」, but you would likely find the dictionary form of the root「さらふ」in a Classical Japanese dictionary.
In the same way as "襲わる" with "襲われる - be attacked", "奪わる" with "奪われる - be robbed of," "囚わる" with "囚われる- be captured," “さらわる” is a bit oldish way of saying “さらわれる,” which is a passive form of the verb, さらう, and you can put Kanji character, “攫う” to this.
Kenkyusha's Japanese English Dictionary (英和中辞典：5th Edition) defines “攫う” as:
横合いから持ち去るーcarry away, sweep away.
“ハロルド王子さらわる” means Prince Harold was kidnapped.
There is another Kanji character, “浚(う)” to “さらう,” which means:
（川、井戸などの）底にたまっているものを取り除く‐clean out (a well) dredge (a river). -Readers Japanese English Dictionary.
攫う and 浚う are different words though pronounced the same.