What is the ruling on praying behind an Imam who commands the slaughter of a black hen, anointing with its blood, and throwing it in an open field, and is known for writing talismans and amulets?

You must verify before judging the Imam. If it is confirmed that he asked the patient to slaughter a black hen and smear herself with its blood, then there is a detailed explanation:

1. If he believes that the blood of a black animal has an effect on removing diseases: He is ignorant, and this is minor shirk because he is affirming a cause that is neither শরعي (religiously prescribed) nor kawni (naturally established). He should be advised and taught. 2. If he intends the slaughter for the jinn, or mentions a name other than Allah’s with His name: He is committing major shirk, and it is likely that he is a sorcerer. One should not pray behind him; rather, people must be warned against him.

The Permanent Committee has issued a fatwa stating that slaughtering with such a belief, or for a specific color or time, is impermissible because it is based on a corrupt belief. If it is for the jinn, it is major shirk. Those who do so must be advised, for Allah alone is the one who removes harm.

Ibn Baz issued a fatwa that going to soothsayers and charlatans is forbidden and among the gravest sins, and it is not permissible to seek healing with impure substances.

As for the Imam's writings:

If they are Quranic verses, Hadith, and legitimate supplications (dhikr): There is no harm. If they are shirk-related or satanic talismans: His action is major shirk, and praying behind him is not permissible.

It is advisable to present his case to trustworthy scholars for advice. If he persists in his actions, people must be warned against him.