She walks north and she sees the wisp of light hovering just inside the room ((I'm assuming Kwiz is following closely behind since it was good suggestion)). With Kwiz's investigation roll, he notices that the rotting wood on the ground where the hallway ends and the room begins looks like a trap.

“Stop!” Kwiz calls out “Below you... there is a trap set there. Though set long ago, and starting to rot, it’s likely still dangerous.”

You aren't getting anything from detect magic.

Andrømeda stops at the warning, looking back with a nod of thanks. Pulling out a javelin, she takes the blunt end and jabs at the boards, ready to move back incase anything popped out of the walls.

The boards fall into a 10' deep hole with a bunch of rusty sword blades set into the ground, pointy end facing up. It would have been impossible to not be skewered had anyone fallen in.

"Setting traps for de unvary. If de sverd is here, den makes sense dir vere steps taken to keep den protected." Andrømeda says, looking around for a way to safely pass the pit.

Perception: 10

or would it be Investigation: 18 ?

It's only about 5 ft wide so you could just hop over.

Kwiz waves his hand in the air and a spectral mage hand appears. He directs it to pat down on the ground just after the hole to test it and ensure it’s solid ground before his friend tries to jump over.

It's solid. 

Rush does his best to keep up with his busy friends.

”Did you say horses? Plural? Sort of odd, right?”

“Likely the retainers for the Lady.” Kwiz replies after confirming the ground ahead is solid. Clearly excited about what’s ahead he adds, “We’ll certainly go check after we see the dragon skull.”

Andrømeda hops over the hole and continues moving forward, cautiously, pausing to jab at the ground in front of her with the javelin between strides.

Andrømeda doesn't find any more traps in the ground, despite her careful prodding. She does pass by one of the sarcophagi as she goes (left? right?) around the center column of rock. When she walks into the open part of the room, she sees what Kwiz saw earlier: a big dragon skull at the end of the room with the bones of a dragon inset into the wall.

As Kwiz walks around the corner, behind Andrømeda, his detect magic starts to ping like crazy. He notices something he hadn't seen before: a longsword is laying on top of the dragon's skull.

((For the sake of reference, it's only been about 3 minutes from when Kwiz cast Detect Magic to know. So, he has 7 more minutes left on the spell.))

Kwiz, finally seeing an ancient treasure that has likely been untouched for far longer than he’d been alive cannot contain his excitement. True adventure! True treasure! This is what he’d left for! He dashes forward saying “This must be it!” And almost tumbles into the dragon skull and he remembers that ancient treasure never comes without a cost and stops in his tracks. Slowing down he hovers his mage hand over the sword and carefully looks ((26)) around the skull and the sword before grasping it with the mage hand if he feels it’s safe.

After a careful investigation, Kwiz feels like it's safe to pick the sword up.

As he does, he can get the familiar tingle of the magic that courses through the sword.

But, no sooner does Kwiz feel the familiar flow of magic than he feels a whoosh of energy come from the direction of the skull right past him and then a small, violent whirlwind of energy forms in the middle of the room. 

From the whirlwind, you hear an elemental female voice harshly demand, "Put that back or be buried with it!"

“AH!” Kwiz cries, and backs up ((north-west, along the wall beside the skull, would be away from the middle of the room, and the surging energy)) as the sword dangles floating in the air by his mage hand. He quickly casts a minor illusion around himself so it appears that he is under a pile of dirt and rubble that would blend in with the cavern around him, and make him unseen ((though not ‘hidden’ exactly)).

The spirit will roll to see whether or not he recognizes Kwiz (DC15 Investigation check): 8 

"This is your last warning," it grumbles.

As the dirt swirls around it, you feel like the creature itself is invisible, roughly a medium sized creature in the way that a gnome or a goblin is, but the swirling dirt around it allows you to get an idea for the creatures form: an armored medium humanoid. It's floating above the surface about 1 foot. 

“Dir is no need for alle de fuss.” Andrømeda says, interposing herself between Kwiz and the spirit. “Vi seek to slay a Wyrm det threatens Nevervinter and den’s lands. Vould Dame Alagondar not vant her Sverd to be used for such a task?”

Roll Persuasion. (It's a high DC because the creature was placed here for a purpose - to protect that sword.) DC15.