Learn Polish: Rozmawialiśmy o sojuszniku, który jest uległy królowi. - We were talking about an ally who is submissive to the king.

The adjective uległy (submissive/yielding) directly governs the Dative case. In Polish, certain adjectives mandate that their object be in a specific case. Just like the verb it comes from (ulegać - to yield/submit), it takes a bare Dative object, meaning no preposition like "to" or "towards" is needed.

Since sojuszniku is in the Locative case, shouldn't the relative pronoun który also be in the Locative?

No, a relative pronoun takes its gender and number from the noun it refers to (masculine singular, matching sojusznik), but its case depends strictly on its grammatical role in its own clause. In the second half of the sentence, the pronoun is the subject doing the action ("who is submissive"), so it must be in the Nominative case (który).

Why does the Locative form of sojusznik (ally) end in -u (sojuszniku) rather than the typical masculine -ie?

Masculine nouns ending in the velar consonants -k, -g, and -ch almost always take the -u ending in the Locative case (e.g., o sojuszniku, w banku). This happens because adding -ie would force a harsh consonant mutation that Polish avoids for these specific noun endings.

Is it grammatically correct to change the word order to który jest królowi uległy?