Learn Finnish: Taikasauva ilmestyy, mikä yllättää yleisön. - The magic wand appears, which surprises the audience.

This is a crucial rule for relative pronouns! You use joka when referring back to a specific noun. However, when "which" refers to an entire preceding clause or idea (in this case, the whole event of the wand appearing), you must use mikä.

Why does yleisö (the audience) end with an -n here?

The -n ending marks yleisön as the total object of the sentence. The action of surprising (yllättää) happens to the audience. Because the sentence is affirmative and the action affects the whole audience, this single object takes the -n ending.

The word for "surprises" is yllättää. Isn't that also the dictionary form?

Yes, they look exactly the same! Yllättää (to surprise) is a Type 1 verb. To make the "he/she/it" form, you normally double the last vowel of the stem. The stem is yllättä-, but Finnish doesn't allow three identical vowels in a row (-äää), so the third-person singular simply stays as yllättää.

What is the dictionary form of ilmestyy (appears)?

The dictionary form is ilmestyä (to appear). It is a Type 1 verb. To form the third-person singular ("it appears"), you take the stem (ilmesty-) and double the final vowel, which gives you ilmestyy.