Derendil quickly follows behind Thunk, clings to the wall and overtakes the orc. He stands before the undead and calls in evish "my lady, how can we be of service?" the cursed prince does not attack, but prepares to defend himself if needed.

The wraith looks at Derendil with disdain and extends a shriveled arm and tries to drain the life from Derendil (to hit: 11) but the prince evades the arm and calls again (elvish) "we came here to help someone in distress. We do not want to fight you." The wraith's face does not change and it prepares to strike again.

Initiative order:
21 Derendil
19 Wraith
13 Drogen
11 Thunk
11 Arokas
  7 Tanis

Drogen goes next.

OOC: Drogen's spiritual weapon is still up there, it will last for 3 more turns. The newly discovered room sits directly below the one with four sarcophagi. It would be possible, but uncomfortable, to lie down into the hole and cast spells into it.

Drogen moves over to the hole, looking in he sees Tanis in the way at the bottom and waits for him to move. He does send his spiritual weapon down and attacks the wraith with it. 

to hit: 11 damage: 10

     Thunk stomps up to the wraith and attacks recklessly, angry he can't find the lady voice of his head!

Reckless Shortsword Attack: 16 or 8 for 8 damage
Bonus Action offhand SS Attack:  12 or 10 for 3 damage

(Reckless Attack so attacks against Thunk have adv til start of his next turn.)

Drogen's spiritual hammer manages to get next to the wraith, but fails on hitting it. Thunk is able to hit the undead with his first attack, but misses his second.

Initiative order:
21 Derendil
19 Wraith (4, in melee with Thunk and Derendil)
13 Drogen
11 Thunk
11 Arokas
  7 Tanis

Arokas goes next.

Arokas forms a cackling ball of energy, tossing a Chaos Bolt at the wraith from up above.

To hit: 18, Damage: 5 acid damage

Movement:  Tanis skirts to the far NW corner of the room using his 40' movement on the first round of combat (Dread Ambusher).  As he goes he tries to speak mentally to the voice in Elvish.  Is this you?  Brysis?  Or are you someone else still imprisoned?  We see nothing but death here.

Aloud in Elvish he cries:  "Derendil!  Open your eyes, my prince.  If indeed this is Brysis, she is corrupted far beyond salvation, seeking only to slay us and likely raise us as more specters like the ones we encountered above.  Or the trapped voice may yet be someone else, but for now, we must fight!"

Bonus Action:  Tanis transfers his Hunter's Mark to the wraith.

Action:  2 attacks due to Dread Ambusher, second one with an extra d8 of damage, both at advantage (I think - invisible to darkvision).

Hand Crossbow attack 1 to hit:  21
Hand Crossbow attack 1 damage plus Hunter's Mark:  5 + 4 = 9 piercing

Hand Crossbow attack 2 to hit:  22
Hand Crossbow attack 2 damage plus Hunter's Mark plus Dread Ambusher:  7 + 1 + 2 = 10 piercing

Arokas' bolt finds the wraith, damaging the undead. A strange glow forms around the sorcerer and Drogen looks in awe as Arokas instantly transforms into a potted plant. Two bolts from Tanis easily find the undead, and his words snap Derendil back into the fight. He slashes with both claws at the wraith (to hits: 11, 8) but only manages to scratch the stone sarcophagus where it came from.

The wraith retaliates against Derendil (to hit: 8), missing its mark by a long berth.

Initiative order:
21 Derendil
19 Wraith (20, in melee with Thunk and Derendil)
13 Drogen
11 Thunk
11 Arokas
  7 Tanis

Drogen goes next.

OOC: Arokas turns into a potted plant until the start of his next turn. While a plant, you are incapacitated and have vulnerability to all damage. If you drop to 0 hit points, your pot breaks, and your form reverts.

Drogen drops into the hole now that the way is clear firing a guiding bolt at the wraith followed up by a swing from his spiritual weapon.

Guiding Bolt to hit: 12 damage: 9

Spiritual Weapon to hit: 11 damage: 4

     "Where is the lady?  The head voice lady??" Thunk asks loudly even as he swings at the wraith.

Shortsword Attack: 18 to hit for 8 damage
Bonus Action offhand Shortsword Attack:  10 to hit for 3 damage

OOC: since Arokas is currently incapacitated, is there anything for him to possibly do?

OOC:  "You turn into a potted plant until the start of your next turn. While a plant, you are incapacitated and have vulnerability to all damage. If you drop to 0 hit points, your pot breaks, and your form reverts."  Never seen that wild magic surge happen before!

But I read that to mean that at the start of your turn, you turn back into Arokas and can act normally on your turn.

OOC: The weird thing is that Arokas isn’t a Wild Magic sorcerer, he’s a Draconic Bloodline Sorcerer. I’m pretty confused as to where that came from.

OOC:  I think it may be some weird thing with ancient Netheril and casting magic in this ancient relic of a place (with the weird golden dust motes and all).

OOOC: That would make sense, but Arokas has casted leveled spells before and nothing has happened. He’s still in the room where we fought the four specters, so that hasn’t changed. If that’s the case, then I’ve gotten lucky and the surges haven’t affected Arokas until now.

OOC:  Yep - and Tanis cast Hunter's Mark and Drogen cast Spiritual Weapon and maybe Guiding Bolt I think?  Might be that on any spell (or leveled spell), there is a 1 in N (maybe 1 in 20) chance of a wild magic surge here.

So anyway, since Arokas turned into a potted plant after his Chaos Bolt last turn, he turns back to Arokas and can take his turn now.

Drogen's bolt misses the wraith by a couple inches, it bounces off the wall on the far end of the tomb and explodes into a full grown unicorn. The celestial beast looks confused at the ongoing fight. Thrown off by the sudded apparition, the spiritual hammer swings at the empty air.

Thunk's sword hits the wraith. A familiar voice sounds in everyone's heads replying to the orc's question "In the sarcophagus! I can help you!"

Initiative order:
21 Derendil
19 Wraith (24, in melee with Thunk and Derendil)
14 Unicorn
13 Drogen
11 Thunk
11 Arokas
  7 Tanis

Arokas goes next.

OOC: every leveled spell in the tomb should have triggered wild magic surges, but I forgot about them until the fight with the wraith. Luckily it makes sense in game that the magical disturbance would be greater near the true tomb. Arokas is back into his normal shape now.

Arokas shifts back into his half-elven form, hyperventilating and shaking with wide eyes. Taking a deep breath, he casts a second level Chaos Bolt at the wraith, but misses entirely.

To hit: 8

I'm not sure if Arokas has taken damage, but he recovers 13 hitpoints due to the wild magic present in the tomb.

Initiative order:
21 Derendil
19 Wraith (24, in melee with Thunk and Derendil)
14 Unicorn
13 Drogen
11 Thunk
11 Arokas
  7 Tanis

Tanis goes next.

Stunned for a moment at the appearance of the unicorn, Tanis recovers in time to addresses them in Elvish, his voice shaky:

"Help us, angelic one!  I know not from where you came, but you plainly see the undead abomination we face."

Then, hearing the telepathic voice, Tanis shouts in Dwarvish to Thunk and Elvish to Derendil:  "The voice again.  Maybe they are still there and can help us, inside the sarcophagus?  I mean the big box, Thunk.  You two are the strongest of us - can you pry it open?"

Staying where he is, Tanis focuses his attacks on the wraith, still invisible to darkvision:

Action:  Tanis attacks the wraith with his hand crossbow (Hunter's Mark already active on target).
To hit (with advantage from being invisible):  22
Damage plus Hunter's Mark:  4 + 3 = 7 piercing

Bonus Action:  Tanis Crossbow Expert bonus attack on wraith.
To hit (with advantage from being invisible):  26
Damage plus Hunter's Mark:  5 + 4 = 9 piercing

Two bolts go through the wraith, making its form waver slightly. Derendil nods at Tanis' words and pries the sarcophagus metallic lid open. It is surprisingly malleable under his hands. Inside the stone sarcophagus, lying atop Brysis's withered and mummified corpse, is a sword handle with no blade (elvish) "there is no one here!"

The wraith turns its attention to the one defiling its grave and lashes at Derendil (to hit: 15, damage: 28), smiling as the life starts draining from the cursed prince.

As the unicorn canters toward the wraith you see her stumble over nothing in the middle of the room. She tries to impale the undead with her horn (to hit: 14, damage: 5). It hits but is not very effective.

Initiative order:
21 Derendil (28)
19 Wraith (35, in melee with Thunk and Derendil)
14 Unicorn
13 Drogen
11 Thunk
11 Arokas
  7 Tanis

Drogen goes next.

OOC: Derendil failed his constitution saving throw and had his maximum hit points reduced.