The Man in Black takes the form of Locke. ("The Life and Death of Jeremy Bentham")
Known shape shifters
The Man in Black
The Monster confronts Ben in the shape of Alex. ("Dead Is Dead")
The Man in Black has the ability to manifest as different people on the Island. Generally it takes the form of a victim's loved one in order to enforce a point. When confronting Eko shortly before his death, the Man appeared to take the form of Yemi, Eko's brother, in order to convince Eko to confess. When Eko refused to confess, however, Yemi walked away. Eko followed, but was almost immediately attacked by the Man's smoke monster form, which threw him around the jungle and killed him. ("The Cost of Living")
When confronting Ben, the Man in Black took the form of his deceased daughter Alex. The form of Alex pinned Ben to a nearby column and threatened Ben with death should he not obey Locke's orders. Ben agreed, convinced by the manifestation of his daughter. ("Dead Is Dead")
Ben kills Jacob.
After the crash of Ajira Airways Flight 316, the Man in Black saw an opportunity to exploit a "loophole" in the conflict between himself and Jacob, and took over the form of Locke, whose body had been in the cargo hold of the plane. Posing as Locke, the impostor convinced the Others to travel with him to the Statue of Taweret in order to speak with Jacob, though in secret he plotted to kill him with the help of Ben. Upon reaching the statue, the impostor confronted Jacob, who recognized him immediately. With the help of Ben, the impostor succeeded in apparently killing Jacob.
Ilana and Bram became aware of the impostor's deception, however, when they found Locke's body in a crate in the cargo hold of the plane. ("Dead Is Dead") Ilana and Bram, along with several other survivors, followed the others to the statue, where they revealed the impostor's deception to Richard Alpert. ("The Incident, Part 2") Ilana's men were subsequently killed by the imposter when he transformed into the Monster, and began to manipulate potential "recruits" to help him in his goal of going "home." ("LA X, Part 2")
It wasn't long before everyone else involved in the war became aware of this deception.
- The twist that the reborn Locke was actually Jacob's enemy was not revealed until the character had appeared for several episodes, making it a plot twist. ("The Incident, Part 2") This twist also played off the assumptions of many viewers that Locke would be resurrected upon returning to the island. The revelation that Locke's imposter was also the Monster was another plot twist. ("LA X, Part 1")