Learn Finnish: Taikuri löysi taikasauvan, mistä seurasi uusi temppu. - The magician found the magic wand, from which a new trick followed.

We use a form of mikä instead of joka when the relative pronoun refers to the entire previous clause. Here, the new trick followed from the whole event of the magician finding the wand, not just from the wand itself.

Why is the relative pronoun in the elative case (mistä)?

The verb seurata (to follow, to result) requires the elative case (ending in -sta or -stä) when you are talking about what a consequence stems from. Literally, it means a new trick followed from that event.

Why does taikasauvan end in -n?

It is the total object of the verb löysi. Because the magician found the whole wand and the action was completed, the object takes the genitive-like ending -n.

Why is the subject uusi temppu at the very end of the sentence?

In Finnish, the word order often shifts to put the newest or most important information at the end. The structure mistä seurasi (from which followed) is very commonly followed by the subject to introduce the consequence as the grand finale of the sentence.