The Saptâin were Númenorean Magicians and Seers.They were heirs of the Wisemen of the Edain and pupils of the Elves who watched the Stars and brewed strange potions, thus they were in fact mortal Wizards who had delved into Astrology and Alchemy and other deep knowledge.
The Seers of Arnor were descendants or successors of this Order as were the Wizards of Umbar and the Blood-Ring of Minas Tirith.