Grimnir avoid the attack and responds with a brutal mace swing.

On S4: 20 damage bludg 7 + 1 Fire damage
On S4:20 damage bludg 8 + 1 fire damage

Using another Ki point to Dodge (bonus action)... Attack against Grimnir at disadventage

Jimjar stays far from the reach of these creatures and releases 2 arrows.

on S3 shortbow1: 19 damage pierc 7
on S3 shortbow2: 13 damage pierc 5
on S3 sneak attack: 7
Then he tries to hide: sneak 8

I try another frostbite, mainly because there's next to nothing else I can do.  2 damage, after resistance. 

Oorr will keep using the magical dagger against S4 (eventually S3). Oorr loves his dagger.

Attack: 12 Damage: 7
Attack: 8 Damage: 8

Zephyr Strike 8 more rounds.

Valrish swings at S3 with his axe while raging and while Eldeth slashes at S2:

Valrish:
Attack: 15 Damage: 14
Attack: 10 Damage: 12

Eldeth:
Attack: 14 Damage: 4
Attack: 7 Damage: 4

Con Save for Frostbite: 18

The wails of the specters echo through the tomb as dagger, axe , and mace swing through the undead.  The final specter continues its assault on Eldeth.
Life Drain on Eldeth: Attack: 17 Necrotic Damage: 11

Eldeth shudders as the specter drains the life from her, reaching through her shield to grip her heart. 

Round 2
20 Oorr/Zippy/Rumpadump
14 Specters (S1: 0/22; S2: 19/22; S3: 0/22; S4 0/22)
12 Valrish/Eldeth
09 Belwar/Derendil
07 Grimnir/Jimjar

Oorr will keep using the magical dagger against the last specter. Oorr loves his dagger.

Attack: 23 Damage: 7
Attack: 9 Damage: 5

Zephyr Strike 8 more rounds.

Valrish and Oorr strike the final blows against the last specter, silencing its wails and preventing it from draining the life completely from Eldeth, who now appears weakened, aged, and cold. She shudders a bit as she stands.

The grinding of stone can be heard from the steps leading down to the hidden chamber below. Then a sudden thud, as stone falls on stone. A cackling low voice calls from below. "Hahaheheha! Your deaths will free me from this prison you disturbers of the tomb of Brysis of Khaem! Accursed are you most miserable creatures!" 



Another but familiar voice calls out to your minds. "Below, in the sarcophagus, I can help! She is dangerous, be careful!"

Oorr is very concerned about Eldeth's status: "Eldeth, stay here with Zippy and Rumpadump. They will keep you safe."

"It seems there are two entities down there. I can't say of their intentions, though." With that said, Oorr starts leading down the tunnel.

Grimnir is having a quick look at Eldeth’s wound. Does it seem reversible ?

medecine: 20 (if arcana my modificatif is -1)

Then Grimnir and Jimjar follow Oorr, trying to stay hidden

stealth grimnir: 19
stealth jimjar: 23

"I don't really like the sound of going down there, but it doesn't seem like we've got much of an option." Valrish says as he follows Oorr.

Quote from RonanKP >>

This post has potentially manipulated dice roll results.

Grimnir is having a quick look at Eldeth’s wound. Does it seem reversible ?

medecine: 20 (if arcana my modificatif is -1)

Then Grimnir and Jimjar follow Oorr, trying to stay hidden

stealth grimnir: 14
stealth jimjar: 19

Eldeth nods weakly to Oorr as Zippy and Rumpadump watch over her. Grimnir, you discover that she seems to be okay, and likely just needs to rest for a good while before the effect of the specter's life drain wears off. (OOG The effect lasts until she takes a long rest.)

You descend into the hidden tomb, and your eyes sparkle with the reflected light of a gems inlaid in the walls. A gilded sarcophagus sits at the Northen Wall. It is open, and a golden lid lies on the floor beside it. This must be what made the large thumping sound you heard. In the sarcophagus you see the mummified corpse of a woman, dressed in fine linens of purple, gold, and black, but covered in dust. The body is clutching something gold and shining to its chest. It's difficult to see what it is, as above the sarcophagus is floating a malevolent black spirit. It speaks with the deep voice you heard above, "You will be my salvation from this place, and be my servants in death!" And it flies directly at Oorr.

Wraith Initiative 11 

(OOG Since this is directly after the last fight, I'm going to keep the same initiative to keep things moving. I will also assume your characters are reasonably certain of the resistances due to the last fight with similar creatures.)

Round 1
20 Oorr/Zippy/Rumpadump
12 Valrish/Eldeth
11 Wraith
09 Belwar/Derendil
07 Grimnir/Jimjar

Oorr beats his wings to cast Zephyr strike once again. He then flies over the sarcophagus and sees inside as the voice suggested.

Quote from filcat >>

Oorr beats his wings to cast Zephyr strike once again. He then flies over the sarcophagus and sees inside as the voice suggested.

The wraith shrieks as Oorr ignores it to rush to the sarcophagus. Up close, you can see the treasure littered in the grave around the mummified body. The soft voice speaks once more, "Don't waste any time, pick me up and we'll destroy her together." The voice is in your head, but you think you understand now that it's coming from the object in the grip of the corpse. On closer look, it appears to be a gilded sword hilt. It gleams as if new, unlike all of the other dusty objects in the sarcophagus and room. 

"Don't touch that!" screams the wraith, as it lunges at Oorr.

If there is something that Oorr hates the most, it is pure evil and that ghost-like creature seems to be such. If picking the hilt makes the ghost furious...

Oorr picks the hilt.

Oorr, the hands of the corpse crack and break as you pry the hilt free. It is light in your grip, and you feel a presence within it as you realize this is what has been speaking to you telepathically. The presence is kind, but fierce, and you feel its enmity directed at the wraith. "Thank you, friend. Now let's send this evil back where it came from!" As soon as the words enter your mind, you feel a slight vibration and buzzing from the hilt. A light like the sunrise appearing over the horizon erupts from the hilt, forming a long blade of radiant energy. The room brightens, illuminated by the blade as if it were the sun. The wraith recoils from you, shrieking.

Round 1
20 Oorr/Zippy/Rumpadump
12 Valrish/Eldeth
11 Wraith
09 Belwar/Derendil
07 Grimnir/Jimjar

Oorr feels the power of that sword with all his feathers. He first a telepathic thanks to the sword, then loudly says: " My friends and I, Oorrof the Last Aerie, will destroy you, evil being. "

The wraith flees from the sword, away from Oorr, and towards the entrance to the tomb. It passes through Valrish and Grimnir, causing them to shiver, but take no damage. Away from the brightest light shed from the sword, the wraith stops near Belwar and reaches out to touch him with a shadowy claw.

Wraith Life Drain on Belwar: Attack: 16 Damage: 24

Life Drain. The target must succeed on a DC 14 Constitution saving throw or its hit point maximum is reduced by an amount equal to the damage taken. This reduction lasts until the target finishes a long rest. The target dies if this effect reduces its hit point maximum to 0.

As the ghost-creature turns around to get to the others, Oorr, wielding the sword with both hands, strike at it, the blow guided by the wind magic.
Opportunity Attack: 15 Damage: 13

As the wraith pass by Grimnir to attack the wizard, Grimnir swing with his mace
Attack of Opportunity (magic mace): 9 damage bludg 5 + 1 Fire damage

Grimnir then runs to the undead, flanking the creature and unleash a series of blows with his mace:
20 damage bludg 5 + 1 Fire damage
12 damage bludg 5 + 1 fire damage

Using a ki point to DODGE (bonus action)

Seeing what can happen at close range, Jimjar stays far from the creature and releases 2 arrows.

shortbow1: 18 damage pierc 5
shortbow2: 8 damage pierc 4
sneak attack: 5