ennuager

A rare, poetic verb meaning to cloud over — to cover the sky (or figuratively, a mood or expression) with clouds. You'll meet it in literary description more than in ordinary speech, which prefers se couvrir de nuages.

• Le ciel s'ennuage à l'approche de l'orage. — The sky clouds over as the storm approaches.
• Un voile de tristesse ennuageait son regard. — A veil of sadness clouded her expression.
• Le soleil disparut, ennuagé peu à peu. — The sun disappeared, gradually clouded over.

Rare enough that there's no genuine common-mistake pattern — it's a literary choice over the everyday se couvrir de nuages, not a word learners need for practical speech.