The horse feels the Riders coming back, of course.  It shouldn't have hoped to get away from them.

The wraiths let Gord run away.  

Or, they can't see him, but they can see the mysterious hole in the Song of the world (which looked like a bag to their horses) moving, and they're guessing someone they can't see is carrying it. Or even if it's moving on its own, they don't care.  The Ring is right there.  And that's what their Master told them to get.

They still don't know what mysterious magic blinded them to all the people who had been there a moment ago... but they don't want to wait and let things get even stranger.

Frodo is wrestling with this sudden wave of fear too, but his fear is swallowed up in a sudden temptation to put on the Ring.  

Despite all the warnings he has heard, he can think of nothing else.  It isn't a hope of escape; it isn't a hope of doing anything whether good or bad - he simply feels he must take the Ring and put it on his finger.

He can't speak.  He shuts his eyes and struggles for what feels like a long while... but then resistance becomes unbearable and he draws out the Ring and slips it on.

It's maybe four rounds before Gord hears a hiss - three eager hisses.

And then Frodo's voice - with a strange tone to it - crying in what doesn't sound like the Common Tongue at all, "O! Elbereth! Gilthoniel!"

And then the Hide from Undead spell fails.

Gord's comprehend ran out long ago, so he's going to ignore any cry that's not "O! Gord!". 

But he can see the spell on himself wink out. He knew this would happen, when the wraiths attacked the party before they could scatter, but he really hoped it would buy him more than a few rounds. Now he has to start sneaking away, rather than running pell-mell through the darkness. 

He still doesn't know what senses the wraiths have at what range, and horses can hear in the dark perfectly well; presumably they still have their own horses nearby, besides the one he brought to the party. There might even be more wraiths around; he expected two but but saw three just now, and Strider said there were nine in total.

What can he see if he looks back? The wraiths are practically invisible at range in the darkness, even wearing their cloaks, but Strider at least had a torch going.

He - or someone - is stabbing with the torch like a weapon.

And it looks like the enemy's retreating?

The wraiths didn't kill the party before; they're driving it towards some mysterious purpose. It makes sense they won't kill it tonight, either. That only increases the chance that they'll be looking for Gord, hungry to take their vengeance. Or just hungry.

He's going to sneak away as best he can, while scanning all around him with detect magic running and also keeping an ear out for any sound of horses. He'd like to keep moving all night and find a secure resting place in the morning, after praying. He went from morning in Golarion to evening here; he can keep going unless the drain is much worse than he thought it is, and he has one more lesser restoration he can use.

Behind him, he hears a call of "Frodo!"

And a couple rounds later, he sees three wraiths quickly approaching him from the direction he left the party.

But a moment after he sees them (at the limit of his Detect Magic range), they've dodged off again away from him.

He can't target them if he can't see them! 

He will keep walking downslope and away from the hill, very cautiously.

His caution keeps him from tripping in the night.

He doesn't come across anyone else, or anything unusual for a hill.

Then it's back to plan: spend all night running walking away!

A little ways away from the hill, he sees a pony that looks up at him hopefully as he runs by.  If he looks closely, it's got a bridle on.

...the halflings mentioned something about looking for a pony, didn't they?

He can't ride a pony all night in the dark without exhausting and probably laming it - well, he has cures if he lames it, but he doesn't think it will help him to ride it. It will probably make his tracks easier to follow, and for all he knows the wraiths can use its senses. Anyway, it isn't his and it has their stuff on it.

It must be scared, alone in the night with wraiths around, but Gord can't do anything to help with that. Onwards into the night he goes.

The moon soon rises to light Gord's steps over the uneven ground.  There's a stone-lined narrow path running up and down the dales, if he chooses to follow it.

For as long as he chooses to walk, no wraiths are in sight - nor any other magic.

Meanwhile at the camp, just after the Black Riders withdraw and the fear leaves the hobbits, they're horrified to find Frodo vanished.

Sam is the first to run around the hollow looking and feeling for him.  "D-did Gord put a spell on him too?" he grumbles.  "Make him invisible to us?"

"I hope the Riders didn't carry him away..."  Merry ventures nervously.

"Gord?" he calls.

"If he called on Elbereth, I would hope he was stronger than..." Aragorn murmurs, and then shakes his head because it's not clear what he's hoping.  Is he hoping Frodo was too strong to put on the Ring?  But then his disappearance must mean the Ringwraiths carried him away.  Is he hoping Frodo was too strong for that?  That's a vain thing to hope.

He wishes Gandalf had been here.  Or Glorfindel, or Erestor... or that Gord had stayed.  He couldn't fight the Ringwraiths by himself, but he'd tried his best... and he can only hope that's good enough, for Frodo and for all Middle-Earth.

He joins the hobbits looking for Frodo.  It might help.

It's Sam who literally stumbles over Frodo, lying as if dead, face downwards on the grass with his sword in one hand and his other hand curled around the Ring.

Sam picks him up and carries him over by the now-half-dead kindling where they'd been starting a fire.  "Thank goodness he's here!  But - is it the Black Breath again -"

Strider bends over him and feels his side and shoulders, carefully not touching the hand that's still clutching the Ring.

There's a wound on his right shoulder.  Aragorn frowns gravely.  "Worse, I fear.  Heat water; bathe his wound.  I must be away to see how far the wraiths have gone - and to search for a better balm for his wound."

And then, barely waiting for Sam to nod, he's away into the night.  The moon will rise soon; he trusts the starlight and his (not-)feeling the wraiths' presence until then.

As he satisfies himself the wraiths aren't lurking anywhere nearby, he considers where to go.  Normally, he would search for athelas for Frodo's wound; there should be some growing near here though he didn't see any earlier today.  Or, he could track Gord - whose magical healing must be even better than athelas, at least in the short run.  But Gord fled in the middle of battle; would he want to come back to them?

Or maybe Aragorn can find both tonight...