ぞ is a strong sentence-ending particle. It adds a firm, spoken “I’m telling you,” “listen up,” or “no argument” feeling.
It often sounds masculine, forceful, and authoritative. The speaker usually sounds equal to or higher than the listener.
Common uses include self-encouragement, as in よし、いくぞ, warnings, and strong assertions where the speaker sounds confident and expects the listener to accept it.
You will hear ぞ often in anime, manga, games, and rough casual speech.
Use it carefully in real life. With strangers, teachers, bosses, customers, or anyone you should be polite to, ぞ can sound rude, aggressive, or bossy.