Learn Spanish (Latin America): El guerrero, cuyo linaje es un secreto, ganó la batalla. - The warrior, whose lineage is a secret, won the battle.

The word cuyo translates to 'whose', but it acts like a Spanish adjective. This means it must match the gender and number of the thing being possessed (the lineage), not the person who possesses it (the warrior). Since linaje is masculine and singular, we use cuyo. If we were talking about the warrior's swords (las espadas), it would be cuyas espadas.

Is it possible to use de quien instead of cuyo to say 'whose lineage'?

Not in this exact structure. While de quien means 'of whom', you cannot place it directly before a noun to mean 'whose'. When you have a noun immediately following 'whose', you must use cuyo, cuya, cuyos, or cuyas.

I thought we drop words like un after the verb ser (es). Why is it es un secreto?

You are likely thinking of the rule where we drop the indefinite article for professions, nationalities, or religions (for example, saying él es guerrero instead of él es un guerrero). But secreto is a regular noun here, so we keep the article just like in English.

Why is the verb ganar in the preterite tense (ganó) rather than the imperfect (ganaba)?