Learn Russian: Это семейная линия, историю которой я ищу. - This is a family line, the history of which I am looking for.

Которой is the relative pronoun meaning "which". It is in the Genitive case here because it means "of which" (or "whose"). It takes the feminine singular form to match линия (line), the noun it refers back to in the first part of the sentence.

Why does историю come before которой? In English we say "whose history" or "the history of which".

In Russian, when you express "the [noun] of which", the noun being owned (историю) usually comes right before the relative pronoun (которой). This keeps the two related words together at the start of the clause.

Why is the word for history spelled историю and not история?

Историю is in the Accusative case because it is the direct object of the verb ищу (I am looking for). Even though it is placed before the subject (я) and verb, it is still the thing that is being searched for.

What is the infinitive form of the verb ищу?

The infinitive is искать (to look for, to search). It is a slightly tricky verb because the ск in the stem changes to щ in the present tense, giving us я ищу.