A Darkling is a conjured spirit that can take on the form of any dead being. Seeing as how the power generated to summon a Darkling can be felt by the other Sons of Ipswich, it can be assumed that you have to be an Ascended Warlock to be able to summon one.
The only known summoner of a Darkling is Chase Collins. He's the descendant from the Putnam family that was believed to have died out. He used the form of a male student that he killed at the Dells for the Darkling he summoned.
The only known Darkling seen in the film is the one that takes the form of the male student that Chase killed at the Dells. The student had seen Chase performing magic (also known as The Power), so Chase killed him when he got too curious.