Thunk isn't sure which specter is which but one of them hit him so Thunk strikes back at that one!  (Not sure if it was S2 or S3...) Angered, Thunk rages and then attacks recklessly!

Reckless Shortsword Attack:  17 to hit for 7 damage

Due to attacking recklessly, attacks against Thunk have advantage...

OOC: it was S2 the one that hit Thunk.

Thunk's shortsword finds the specter. Less effective than against a corporeal foe, it manages to hurt the undead. The spectre retaliates against the orc (to hit: 22, damage: 9) draining some of his life away. The one hit by Tanis' bolts flies in the direction the proyectiles came from, going across Tanis' position and flailing at the ranger on its way (to hit: 12), failing to find its invisible foe and ending its movement over the nort east sarcophagus. The last spectre, still turned, flies as far as possible from Drogen. It starts to sink under the floor behind Thunk.

Derendil slashes again at the spectre in front of him (to hits: 19, 12, damage: 5), damaging it slightly.

Initiative order:
17 Tanis (8)
17 Thunk (17)
   9 Three Specters (24) (5, in melee with Thunk and Derendil) (5, turned, in melee with Thunk) (19)
   5 Derendil
   4 Arokas
   1 Drogen

Arokas goes next.

OOC: Thunk has the chance of making an opportunity attack against the spectre that is running away, and has to make a DC 10 Constitution saving throw or have their hit point maximum reduced by 9 by the specter's Life Drain.

Arokas points the fingers of his left hand (minus the middle finger) at the specter attempting to sink away, and shoots a second level Magic Missile. (He uses a sorcery point to reroll 3 of the dice, since I rolled the minimum possible)

Damage: 11+4=15 force damage

Thunk is hit and takes 9 damage, necrotic not a type that his rage reduces...  DC 10 Con save:  13 so his HP Max stays 27 for now...

     Thunk doesn't know about turning and unfortunately has no reason not to take an opportunity attack versus the fleeing specter...

Shortsword Opportunity Attack vs fleeing Specter: Nat 20 for 26 to hit for 13 damage 11 plus 2 from rage...

Thunk hits the spectre before it goes completely underground. In turn it attacks the orc (to hit: 8) but is unable to find its target. Arokas moves forward to get a clear view at the ongoing fight. A well aimed barrage of magic missiles finishes off the spectre that Thunk just prevented from running away.

Initiative order:
17 Tanis (8)
17 Thunk (17)
   9 Two Specters (24) (5, in melee with Thunk, Derendil, and Arokas) (26) (19)
   5 Derendil
   4 Arokas
   1 Drogen

Drogen goes next.

Drogen moves south one square at fires a guiding bolt at S4 (to hit: 7 damage: 16) then BA attacks S2 with his spiritual weapon (to hit: 22 damage: 6)

Realizing too late that his attacks ruined Drogen's turning of the undead, Tanis tries to remember the lesson learned for next time.

Assuming S4 is still up, he shoots another bolt, with his Hunter's Mark still on it:

Action:
Hand crossbow to hit S4 (at advantage due to Umbral Sight):  26
Hand crossbow plus Hunter's Mark damage on S4:  9 + 4 = 13 piercing

IF the bolt finishes off S4, Tanis uses his Bonus Action to attack S2 (without Hunter's Mark) using his Crossbow Expert feat.
OR IF S4 is still up, Tanis uses his Bonus Action to shoot it again with Hunter's Mark, using his Crossbow Expert feat.

Bonus Action:
Hand crossbow to hit S2 (or S4 if it is still up):  23
Hand crossbow, possibly plus Hunter's Mark damage on S4:  9 + 4 (exact same) = 13 more piercing on S4 if it is still up, OR 9 piercing on S2.

Movement:
If S4 is gone, Tanis moves back south 15' and glances to make sure there is nothing lurking in the space he had noticed under the SE sarcophagus.

Drogen's guiding bolt misses the specter next to Tanis, but his spiritual hammer smashes the one in front of Thunk. Tanis' first bolt finishes off the specter next to him, and his second damages the last one remaining. The space under the sarcophagus on the south east corner is dark, looking down Tanis notices that there is an opening in the floor below it.

Initiative order:
17 Tanis (8)
17 Thunk (17)
   9 Specter (24) (15, in melee with Thunk, Derendil, and Arokas) (26) (25)
   5 Derendil
   4 Arokas
   1 Drogen

Thunk goes next.

     Seeing just one specter left, Thunk attacks recklessly!  (adv for him, adv for others against Thunk til his next turn)

Short Sword Attack:  24 to hit for 8 damage...

Bonus Action Offhand Short Sword Attack:  11 to hit for 6 damage

Two hits from Thunk shatter the specter's ethereal form. The ensuing silence is only broken by some heavy breaths after the effort.

OOC: combat is over, we are out of initiative order.

"Well," Tanis strained voice comes from the darkness in the southeast corner, "That was unexpected.  Though perhaps we should have expected it.  Is everyone ok?  And does anyone still hear the voice?  There is an opening under this one sarcophagus in this corner.  Maybe we can move it?"

He pauses.  "Or maybe not if we decide Arokas is right and we don't want to chase this voice any further after what just happened."

     "Thunk can move it," Thunk assures, and heads over to the sarco... sarcough... sar-cough-ugg... The stone thing.  "Wasn't listening to head.  Hello, lady??"

Athletics to try to move sarcopgagus: 11

“I’d just prefer whatever we do to have a little bit of caution,” Arokas says, glaring at Thunk. “I do not wish to get killed because of someone else’s recklessness.”

Thunk drags the sarcophagus away from the wall. It rolls over stone cylinders revealing a four foot square hole in the floor. Something metallic rattles inside the coffin as it moves.

The hole connects to a hidden room below, as big as the one with the four sarcophagi. The room is dark, but you can see that the walls have been decorated with murals full of inlaid stones. A gilded sarcophagus stands atop a stone bier along the east wall, at least twice as big as the ones were the spectres rested. Derendil looks over Thunk's shoulder and comments (elvish) "that truly looks like the tomb of a honored hero."

     "What is... wreck... wrecklessness?" Thunk asks even as he is shifting the sarcophagus away.  "But do not worry, Thunk protect you!"

     Thunk climbs right down into the room, a little more carefully than before now that he has been attacked but still heading straight in.  "Voice Lady?" He asks aloud.  "You still here?"

The voice that called you for help does not respond. But as soon as Thunk's feet touch the ground in the lower chamber, a wraith arises with murderous rage from the gilded sarcophagus. A disfigured elven face looks at you with a mixture of loathing and envy and charges forward.

Time to roll for initiative!
Wraith: 17
Derendil: 12

OOC: feel free to decide if you start the fight on the upper room or down with Thunk and the wraith.

Tanis Initiative:  7

Despite wanting to heed Arokas' warning against rushing in, as soon as Thunk committed to descending, Tanis would have slid down beside him as well.

"What a surprise," the ranger mutters sardonically...

(Still invisible to darkvision if in darkness.)

Arokas stays above where he can see the group but still cast spells. “Of course it was a trap,” he mutters as he sees the wraith appear from the sarcophagus.

Initiative: 11

(common) "No...don't go opening it..." Drogen calls out to late and watches the pair climb in.

Initiative: 13

Thunk initiative: 11