What was the second prophecy in Macbeth?

The witches’ second prophecy is that Macbeth will be Thane of Cawdor. 

After the battle, Macbeth and Banquo hear prophecies from three mysterious witches. They are women with beards and are known as the “weird sisters.” The witches tell Macbeth three prophecies in Act I, Scene 3:

First Witch

All hail, Macbeth! hail to thee, thane of Glamis!

Second Witch

All hail, Macbeth, hail to thee, thane of Cawdor!

Third Witch

All hail, Macbeth, thou shalt be king hereafter!

Macbeth is currently the Thane of Glamis, and becoming Thane of Cawdor would be a promotion. Macbeth will get this promotion because he defeated and killed the last person to hold that title in the battle. When Macbeth finds out the witches were right about the promotion, he is surprised.

ROSS

And, for an earnest of a greater honour,
He bade me, from him, call thee thane of Cawdor:
In which addition, hail, most worthy thane!
For it is thine.

BANQUO

What, can the devil speak true?

MACBETH

The thane of Cawdor lives: why do you dress me
In borrow'd robes? (Act I, Scene 3)

The Thane of Cawdor promotion is not enough for Macbeth. He wants to be the king. That was the third prophecy, and Macbeth now wants it to come true like the others have. This seems less likely when King Duncan announces his son, Malcolm, will succeed him on the throne. This prompts the beginning of Macbeth's path to destruction.

Before the witches' intervention, Macbeth seems to have been a loyal soldier. The prophecies make him greedy. When one came true and the other seemingly did not, Macbeth became upset. He decided he was going to become king, one way or another. He would have to kill in order to make it happen, but that didn't matter to him.