Learn Russian: Таков суровый закон природы: кто-то должен погибнуть. - Such is the harsh law of nature: someone must perish.

Таков is the short, predicative form of the demonstrative pronoun такой. You use it when it acts as the main predicate of the sentence, meaning 'such is'. Because it refers to закон (masculine singular), it takes the masculine form таков. If you used Такой here, the thought would sound incomplete, like 'Such a harsh law of nature...'

Why is кто-то used for 'someone' rather than кто-нибудь?

Кто-то is used when referring to a real, existing entity, even if their identity is unknown. Кто-нибудь is used for hypothetical people ('anyone'). Because the sentence presents this as an absolute, factual law where an actual creature will inevitably die, the certainty of кто-то is required.

Кто-то could refer to any animal or person, so why is должен strictly in the masculine form?

In Russian grammar, the pronoun кто (and all its derivatives like кто-то and никто) is treated grammatically as masculine singular. Any verb or short-form adjective agreeing with it must take the masculine singular form, regardless of the actual gender of who or what it refers to in the real world.

What is the difference between using погибнуть and the standard verb for dying, умереть?