Learn German: Ihr müsst euch sofort abtrocknen! - You have to dry yourselves immediately!

The subject of the sentence is ihr (you, plural informal). The verb sich abtrocknen (to dry oneself) is reflexive here, meaning the action reflects back on the subject. euch is the accusative reflexive pronoun that exactly matches ihr, translating to "yourselves".

I thought abtrocknen was a separable verb. Why hasn't ab moved to the end?

You are right that abtrocknen is a separable verb! However, this sentence uses the conjugated modal verb müsst (have to). A modal verb always pushes the second verb to the very end of the sentence in its full infinitive form. When a separable verb sits at the end as an infinitive, its prefix stays attached.

Does abtrocknen always need a reflexive pronoun?

Not always! It depends on what is being dried. Because you are drying yourselves, it is reflexive and needs euch. If you were drying an object instead, like the dishes (die Teller), you would drop the reflexive pronoun and just use it as a normal verb: Ihr müsst die Teller abtrocknen.

Why does euch come before the time word sofort?