If you like both PJO/HoO and KC then you'll know what I'm talking about.
Rick Riordan doesn't tell us directly but he gives us a few hints. Read on if you have the same idea or is intrested in a knew theory.
WARNING: This wil contain spoiliers!
When you read KC's Shadow's Serpent, then you'll reconize these quotes.
Near the end of the book Sadie talks with her mother after (spoiler alert) she and her brother defeated Apophis and is visiting there father/Osiris. Her mom says," But there are other challenges as well. I haven't completely lost the gift of prophecey, even in death (her mother's dead... *gasp*). I see murky visions "of other gods and rival magic"."
So, it's pretty much talking about the Greeks and the Romans. Don't believ me? Maybe this will back me up.
Sadie says in the third to last paragraph, "We even got reports of unexplainable magic as close as "long island sound"."
Long Island Sound? That's the location as Camp Half-Blood! If you still don't believe me... than you're an idiot.
PJO: Percy Jackson and the Olypmians
HoO: Heros of Olympus
KC: Kane Chronicles