Learn Finnish: Älä poistu, ja älköön kukaan loihtiko mitään tämän piirin takana. - Do not leave, and let no one conjure anything behind this circle.

You need two parts. First, the negative verb takes the 3rd person imperative form, älköön. Second, the main verb drops its infinitive ending and takes the suffix -ko or -kö. For the verb loihtia (to conjure), the stem is loihti-, making the final word loihtiko.

Why does the sentence use both älä and älköön for its commands?

They address different grammatical persons. Älä poistu is a standard 2nd person singular command directed at "you" (do not leave). Älköön kukaan loihtiko is a 3rd person command directed at "anyone" (let no one conjure), so it requires the 3rd person negative verb älköön.

Why do we use kukaan and mitään here instead of words like joku and jotain?

The clause is negative, governed by älköön. In Finnish, affirmative pronouns like joku (someone) and jotain (something) change to their negative counterparts kukaan (anyone/no one) and mikään (anything/nothing) in negative sentences. Mitään is in the partitive case here because it is the object of a negative action.

Why does the sentence use takana instead of taakse for "behind"?