Spellbound describes a person whose attention is completely held — listening or watching so hard that they forget everything else, including moving. The children sat spellbound as she told the story. The audience was spellbound. The image is the same magic one: a spell has been cast, and the person is bound, tied, unable to move away. That is why it so often appears with words about stillness — sat spellbound, stood spellbound, held them spellbound, watched spellbound. It goes with people (or animals, or a crowd), never with the thing doing the holding: the story is spellbinding, the listeners are spellbound. Grammatically it is a bit narrow, which trips learners up. It sits happily after a verb — she was spellbound, they sat spellbound, the speaker held us spellbound — but it sounds odd in front of a noun; "a spellbound child" is possible but rare, and most writers would recast the sentence. There is no verb form you will need: nobody says "I spellbound them", they say "I held them spellbound". The feeling in the word is wonder, not fear. Someone spellbound is enjoying it, or at least amazed, so do not use it for a person frozen in horror — for that you want transfixed, frozen, or rooted to the spot. It is a slightly literary word, comfortable in a story or a review, a little dramatic in casual chat. In everyday Taiwanese classroom English, "they were completely absorbed" or "they couldn't look away" says a plainer version of the same thing.