Learn Finnish: Kuka tahansa voi käskeä tätä henkeä.

Adding tahansa after a question word turns it into an "any-" or "-ever" word. Here, kuka (who) becomes kuka tahansa (anyone / whoever). It is in the basic nominative case because "anyone" is the grammatical subject—the one doing the commanding.

Since kuka tahansa can refer to multiple people, why is the verb voi in the singular?

Just like the English word "anyone", the phrase kuka tahansa is treated as a singular subject. Therefore, it requires the third-person singular form of the verb, the same as hän voi.

Why is tätä henkeä in the partitive case?

The verb käskeä (to command or order) is a partitive verb. When you command someone, the target of your command must be in the partitive case. So, tämä henki becomes tätä henkeä.

I've learned that henki means life or breath. How does it mean genie here?

Henki is a very versatile word whose core meaning is "spirit". Depending on the context, it translates to spirit, ghost, breath, life, a person (as in "price per person"), or a mythological spirit like a genie from a lamp.