"She's climbing in your castles, she snatching the people up, trying to steal them so you need to hide your children, hide the Telmarines, and hides the creatures 'cause she's stealing everybody up here. You don't have to come and confess you're hiding down there, the kids will find you. So you can run and tell that, new witch."

-The Silver Chair: coming soon! (Epic Music) It's not easy being green!

Ok, maybe not.

-Follow the Signs

maybe. or:

-Things are not as they seem

Many years later, Eustace returns

It's not Tilda! it's not Tilda! We promise!

How about "Evil lies beyond Narnia- A quest into the unknown".

"4 signs will guide them to a hidden world, and a vanished prince."

I like yours, Aravis Narnia, but I think they should go with Libby's.

Love your suggestion, Anhun. But Louloudi the Centaur's is hilarious fun!

Yeah, I literally LOLed at Lou's suggestion. Someone needs to make a Narnia remix of that video.

Where, oh where has that little snake gone? Oh where oh where can she be? With her mist so green and her wand so long. Oh where oh where can she be?

But seriously,

Believe only the signs.

^ Anhun gasps in horror as she envisions a "Sing-along with Jill" version of Silver Chair.

"If you expect the Pevensies and the WW to be here, you're in the wrong place (we hope)!"

"She's climbing in your castles, she snatching the people up, trying to steal them so you need to hide your children, hide the Telmarines, and hides the creatures 'cause she's stealing everybody up here. You don't have to come and confess you're hiding down there, the kids will find you. So you can run and tell that, new witch."

I honestly wouldn't mind if that was the tagline.
They could do something about how it seems peaceful above the ground, but under it there's a big danger. Or they could use Puddleglum's famous line, "I'm going to live as like a Narnian even if there isn't any Narnia!"

-Follow the Signs

-Things are not as they seem

Those two are perfect! I like "Follow the Signs" best. I can just hear "Coming to Theaters December 19th, 2014. Follow the Signs" in that deep voice voice-overs always have for that kind of thing.

Believe only the signs.

That one, too, is awesome! I can hear that one in my head as well.

Here's a couple of mine:

Written in the stars

"Oh dear, I do hope it's not late" (after the "coming to theaters December 19th, 2014" in Puddleglum's voice.)

*cues music remix* "She's taking the Narnians to Underworld! She's taking the Narnians to Underworld!

Okay, how about a more serious one now?

"Underground. Under danger. Under a spell that could lead all of Narnia to fall under evil."

Some of these are terribly cliched and others don't make much sense.
What does 'under danger' even mean? The danger is below ground (mostly), not above.

Lilygloves suggestion is probably the best so far:
"I'm going to live as like a Narnian even if there isn't any Narnia!"