There is a form of shrik in my country whereby people gather sand on a platform and recite some incantations in other to be able to predict, see what will happen to someone or know the future. Is it permissible to observe salat behind an imam that practices such?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

What you have mentioned involves claiming to know the Unseen and this is a kind of divination (Kihaanah) and disbelief in Allaah, and it is not permissible for anyone to claim to know the Unseen because claiming this means denying what is explicitly mentioned in the Quran; Allaah says (what means): {Say, "None in the heavens and earth knows the unseen except Allaah, and they do not perceive when they will be resurrected."}[Quran 27:65]

Therefore, it is not permissible to pray behind such a person, and you should advise him in a gentle and soft manner and clarify to him the danger of such an act to his faith and when standing before Allaah on the Day of Judgment.

For more benefit, please refer to Fatwa 15284.

Allaah Knows best.