Learn French: La bête avait l'air effrayante près du repaire. - The beast looked frightening near the lair.

With the expression avoir l'air (to look / to seem), the adjective generally acts like it follows the verb sembler and agrees with the subject of the sentence, not with the word air. Since the subject here is la bête (feminine), the adjective becomes effrayante.

Is repaire the same word as the one used for "landmark" or "reference point"?

No, they are tricky homophones! Un repaire (with an ai) means a lair, den, or hideout, usually for wild animals or criminals. Un repère (with an è) means a landmark, marker, or point of reference.

Why is it près du repaire and not just près de repaire?

The phrase for "near" is près de. Because repaire is a specific, known masculine noun in this context (le repaire), the preposition de and the definite article le contract into du. In French, de + le must always contract.

Why use the imperfect tense (avait l'air) here instead of the passé composé (a eu l'air)?