Definition
al-Kaahin
الكاهن is someone who foretells the future using the help of devils. The plural of Kaahin is Kuhhaan
كهّان
Ruling
If someone goes to a Kaahin while not believing in him, then his prayer will not be accepted for 40 days. If someone goes to a Kaahin while believing in him, then he has disbelieved in what was revealed upon Muhammad.
the Meaning of "Going to a Kaahin"
Going to a Kaahin means that you go in person or you speak to him/her on the phone or the like.
the Meaning of "prayer won't be accepted for 40 days"
The scholars have two opinions. Some say that prayer rewards are not generated at all as a punishment for going to a Kaahin. Others say that prayer rewards are generated but canceled out by the great sin of going to a Kaahin.
the meaning of "disbelief in what was revealed upon Muhammad"
What was revealed upon Muhammad is Quran and Sunnah. Both has mentioned that no one knows the future except Allah. If someone believes in what a Kaahin says, then he has believed that someone other than Allah knows the future. As a result, such a person has disbelieved in Quran and Sunnah. Such disbelief is a major disbelief.