Coriana's lightning does some damage, but both monsters dodge slightly, and the cold from the tentacle barely seems to affect the Wraith as it lashes it. Khessa puts up a barrier as Syllen is Paralyzed, Xanrym's spell hits the Wraith. Theldrin's Fireball goes off and hits the Revenant with full force, while the Wraith seems completely unphased by the blast, dodging out of the way and even the flames that do hit seem to immediately dissipate. Avery has enough movement to step forward and finally put the Wraith out of it's eternal life with a swing of Whisper, then run down the stairs and also hit the Revenant.

Revenant Recharges Haunting Glare on a 5-6: 2

The Revenant attempts to consume first Avery into it's massive form:

Suffocate against Avery Attack: 21 Damage: 7 Bludgeoning and 10 Necrotic. He is also Grappled and Suffocating

Then Khessa:

Suffocate against Khessa Attack: 9 Damage: Unable to parse dice roll. Bludgeoning and Unable to parse dice roll. Necrotic. She is also Grappled and Suffocating

(Ahh sorry, I forgot to apply Disadvantage to that attack so immediately went in and edited it forgetting it could stuff up the outcome. That attack against Khessa was at first a 10, so should be a miss.)

The Revenant has consumed Avery and is now turning on the rest of you. You all are up. 

Coriana runs towards the revenant, doing a controlled tumble down the slope (Acrobatics 19), summoning her blades on the way.
In contrast with their normal dark appearance, they gleam with white light as she slashes at the tower of death.
Scimitar: AC 10, 8 radiant.
Scimitar: Miss, Nat 1.
Scimitar: AC 17, 6 radiant.

Doubtful of its usefullness here, Coriana moves the tentacle close enough to strike.
Tentacle: AC 9, 9 cold damage.

Khessa is horrified to see Avery, who, after bravely running to her side, is absorbed into the undead's abominable body!

Relieved that at least her magical protection has spared her a similar fate, the blonde arcane warrior calls Anator to flutter before her target's eyes for a momentary distraction, then takes advantage of the moment to wildly swing her magical Greatsword at the monster (trying to avoid the area where the valiant knight was 'absorbed'): "Let him go, Mr. Rotting Heap! It's me you have to deal with!"

[[ OOC: Anator's Action: [action]Help[/spell] to give Khessa's attack Advantage. Khessa's Action: Attack with her magical Greatsword. ]]

Khessa's Attack: 13 Damage: 11 (magical slashing)

Quote from Painted_Beyond >>

Khessa's Attack: 13 Damage: 11 (magical slashing)

[[ OOC: Oh, great... Rolled a 6 and a 5... Well, OK, maybe she was a bit too shocked by Avery's fate. ]]

Avery attacks the revenant: Attack: 28 Damage: 25; Attack: 23 Damage: 15

Round 3: Xanrym

Once Avery finish off the Wraith, its shadowy form dissolving into dark wisps as he rushes to Khessa's side, Xanrym releases his concentration on Bane.  Turning his attention to the massive undead amalgamation, Xanrym is also horrified when the mass of zombies reaches out its many arms and absorbs Avery into its bulk.  But he's happy to see that Avery continues to fight on, cutting his way out of the decayed corpses, Whisper's blade gleaming bright in the lantern's light.

Theldrin's fireball seemed effective, so Xanrym decides to use fire to convince the huge monster to move or maybe just retreat back underground.  Not wanting to endanger Khessa, Coriana, or Avery, Xanrym moves about 15-ft south to get line-of-sight to the "back" side of the monster and creates a bonfire.  Then he moves back to the north to take cover behind the stalagmites.

Movement: 25-ft of movement, ending just 5-ft south of where he started.
Action: Cast spell Create Bonfire as shown on the map
  The Graveyard Revenant must make a DC 17 DEX saving throw or take damage: 7 (fire)
Free Action: Stow crossbow.

Concentration: Create Bonfire, round 1 of 10
  While the bonfire lasts, any creature who enters bonfire’s space for the first time on a turn or ends its turn there must also make a DC 17 DEX saving throw or take damage: 6 (fire).

Turn 3:

Action: Guiding Bolt to Hit: 10 Damage: 12 Radiant

Move a little down the hallway

(OOC: @JehanO -Unless Syllen burns another Wild Shape, I don't think that he can use the Luminous Arrow attack as bonus action this round.  From the druid class:

You stay in that form for a number of hours equal to half your Druid level or until you use Wild Shape again, have the Incapacitated condition, or die. You can also leave the form early as a Bonus Action.

Since Syllen was briefly paralyzed and incapacitated, I think that he lost his Starry Form too.)

(OOC: thank you. I forgot about that.)

Theldrin steps around the corner, aiming rays of fire at the Revenant before retreating back into the passageway.

Scorching Ray
Attack: 21 Damage: 14
Attack: 26 Damage: 4
Attack: 18 Damage: 11

Revenant DEX Save: 8

Coriana and Khessa land a combined one strike on the Revenant, while Avery lands two from within, fighting for his life. Xanrym's bonfire burns the Revenant, but Syllen misses. Two of Theldrin's fiery rays blast into the hulking form, and the amalgamation seems to cry out in pain.

Recharge Haunting Glare: 6

Xanrym, Coriana, Khessa, and I suppose Avery, must make a DC 15 WIS Save before each of their turn, a save ending Paralysis. The Revenant then moves to the east a little.

You all are up, the Graveyard Revenant seems hurt but is still fighting.

(OOC: @drumdigby - Since the Grave Revenant moved out of their reach, I guess Khessa and Coriana can also make opportunity attacks if they still have their reactions and they're not paralyzed by the Graveyard Revenant's Glare, right?)

Xanrym - WIS saving throw: 21 (in game log)