The Red Room. They were mentioned in the prophecy:

"Gaea’s vaulted Red Room waits.
Halflings as the final bait."

[And to say he's lost a lot of sleep trying to decipher the prophecy is an understatement. It's safe to assume he knows the entire prophecy by heart now.]

From what Natalie and Percy told us, they're a group of demigods who are recruiting others and feeding those who oppose them to Gaea.