Learn Turkish: Hiçbiri tapınakta ayinden bahsetmemeliymiş. - None of them supposedly must talk about the ritual in the temple.

Let's break it down: bahset- (to talk about) + -me- (negative) + -meli- (must / obligation) + -y- (buffer letter) + -miş (supposedly / I heard that). The -miş suffix at the very end adds the inferential meaning, showing that this rule or obligation is something the speaker heard from someone else, rather than a direct order they are giving themselves.

Why is "about the ritual" translated as ayinden with the ablative (from) ending?

The verb bahsetmek (to talk about / to mention) always requires the ablative case (-den/-dan) for the thing being discussed. Even though we say "about" in English, in Turkish you literally say you are mentioning from the ritual (ayinden).

The English translation says they "must" talk, but the Turkish verb is negative (-me-). Why is that?

This is because of the word hiçbiri (none of them). In Turkish, when you use a negative pronoun like hiçbiri or hiç kimse (nobody), the verb must also be negative to agree with it. So the Turkish literally says "None of them must not talk...", which translates to standard English as "None of them must talk...".

Is bahsetmek a compound verb? What is its root?