The last person we met going by the name of Reek was back in ACoK. He was really Ramsay Snow the bastard of Bolton, in disguise as his former servant. Ramsay was pursued by some enemy or other and swapped clothes with his servant, then went to Winterfell which was held by Theon at the time. Bran and Rickon escaped from Winterfell and when Theon couldn't find them he killed a miller and his family and tarred and hung up his sons to make people think they were Bran and Rickon. That's why everyone thinks Bran and Rickon are dead. After that Theon finds himself in need of more men and Reek says he can get some and goes off. Robb Stark's castellan Ser Rodrick turns up to beseige Winterfell, calling Theon a childkiller. Theon threatens to kill Rodrick's daughter.
Theon is saved, briefly, when a new army turns up and slaughters the Stark army and kills Rodrick. The leader enters Winterfell. Theon is thankful at first. The leader takes off his helmet and it's Reek. Except he reveals himself as Ramsay in disguise, then whacks Theon round the head and burns winterfell. That's the last we heard of Theon; everyone presumes him burned at Winterfell.
At first it's unclear who this Reek is. Perhaps Ramsay's servant escaped after all. But whoever this Reek is, he has been imprisoned and tortured by Ramsay, so that would not make sense. Reek is brought before Ramsay and some other lords, one of whom recognises him as Ned Stark's ward. So *that's* what became of Theon. He is not dead, as is usual for presumed-dead people in ASoIaF. He has been imprisoned for quite a while.
Now Ramsay wants to clean him up and take him along to meet his wife-to-be Arya, who is in fact fake Arya. Theon should be able to recognise her as such (which is, perhaps, why he is being taken along). Either that or Ramsay has more cruelty planned.
A pox on both their houses, methinks; they're all bad guys. I feel bad for the fake Arya girl, though. I wonder if this could be the undoing of Petyr's plans. The Boltons won't be happy when they find out.
