Breakdown of Чудовище будет спать, пока не придёт тот, кто его победит.
кто
who / whoever
его
him / it (acc)
быть
to be
спать
to sleep
Questions & Answers about Чудовище будет спать, пока не придёт тот, кто его победит.
How does the тот, кто structure work here?
This is a correlative pair used to express "he who" or "the one who". Тот acts as a placeholder in the first clause (meaning "...until that one arrives"), and кто introduces the relative clause (meaning "...who will defeat it"). Both are in the nominative case here because they are the subjects of their respective verbs (придёт and победит).
Why is there a не before придёт? Doesn't that mean "until he doesn't arrive"?
In Russian, when пока means "until" and refers to a future event, it is almost always followed by не and a perfective verb. You can think of пока не придёт literally as "while he has not yet arrived." Even though it looks negative to English speakers, it translates simply to "until he arrives."
Why is его used to mean "it" instead of "him"?
The pronoun его is the direct object of победит (will defeat) and refers back to чудовище (monster). Because чудовище is a neuter noun, its base pronoun is оно, which becomes его in the accusative case.
Why does "will sleep" use будет while the other future verbs don't?