<the DC is 18, per the module.  I'd have to find the first module part to see what it says happens on a failure, but I'm feeling zesty, so in the interest of moving things along...>

As Marlyth attempts to discern the runes and sells empowering the stele, she notes a glow welling from within the stone pillar, soo sparks begin to crackle along the surface and in an instant, a jolt of energy discharges from the pillar, dealing 9 lightning damage to anyone within 5ft, (So Marlyth).

Tabaqui tries next (I'm not retconning this) and as he attempts to manipulate the delicate moving parts within the narrow space that circles the column a few niches from its apex, he too begins to see sparks lighting from within.  In a blink, a dozen branching fingers of lasma leap from the crevice to shock the rogue.   11 lightning.

The trap is out of charges.

Red granite supports and details, from rock frequently mined along the nearby cliffs, enliven the gray stones here. Although this corridor is clearly of more recent construction, it struggles against the shifting ground, too. Fitted stones are being pulled apart from each other over years of slow movement.  

The corridor is in considerably better shape at its north end, where a second seal was installed to further prevent intrusions. This slab is better seated in place, requiring a successful DC 20 Strength (Athletics) check to force open.

Weneon will take a quick crack at opening this door

Look for traps: 12

Athletics: 12

Constitution Check: To maintain concentration on Detect Magic: 9

Marlyth attempts to examine the surface of the slab, and the corners of the seal to search for any kind of crack or holes through which she can see the other side of the slab.

"This sepulcher is guarded by advanced level of magic, I wonder if it's intended to protect something of great importance. Weneon, you should be more careful with your actions. Think before you leap."

Intelligence: Investigation 21

"If everyone did that, then who would ever leap?" Weneon asks, puzzled

O magics.  It's a very solid slab.  Anyone else?

The remaining pair of Will o' Wisps are passing through the stone slab and back again in a slow, possibly rhythmic manner.  They are trying to tempt you into exploring the sepulcher's depths.  

<The module doesn't present anything besides the strength check to move the slab, but I'm open to suggestions.  I'll also remind that I never said you shouldn't take a rest - you're welcome to -  I just advise being rid of the WoWs before you do.  I'll also (hopefully) expedite this bit by revealing that the stone slab is in no way trapped.  This whole series does have quite a few traps.  The stone slab is not one. >

Strength: Athletics 15

Marlyth assumes the form of a Jaculi by using her Wildshape feature and attempts to add extra pressure on the slab.

Weneon draws her whip and begins swinging it in the rhythm of the wows.

"Should we get rid of them and take a moment to sleep?"

 I'm feeling like this strength check adds nothing and defies conventional wisdom to the effect of: "if there is no time limit and no risk of failure, don't roll dice".  I honestly kept it because its written there and I didn't think it would pose a real hurdle.  So, given that nothing is actively trying to kill you.

After an half hour of trying, the stone slab begins to move, inch by inch it wriggles free of its position, until finally, you manage to displace the seal with a noticeable thud as it topples to the stone floor.

The passage here is decorated with fine tile in an abstract, dwarven style. Though the stairs have settled unevenly, the passage itself is level and tidy.

Quote from Wile_E_Coyote >>

 

You are in area 9.

"Rest would be welcome," suggests Angus.  "We can eliminate these undead abominations first, or simply take watches as they do not seem inclined to attack while any of us are awake."

Tabaqui volunteers to take the first 4 hr watch, as he does not need the sleep other races do.  

Jaculyth will respond to the question posed by Weneon; she will sway her head side by side which implies a positive or negative answer. She uses the muscles of her reptilian physique to climb up vertically or move horizontally on the wall. Then she attempts to bite one of the Will o' Wisps while attempting to swallow the tiny ball of light.

Jaculi

Attack: Bite X Will o' Wisp

Attack: 13 Damage: 8

("Beep, beep", How many Will o' Wisps are left? Marlyth will need a short rest of an hour, but not more than that.)

Just 2, they're the only things you've seen that you haven't killed.

Despite her speed,, Jaculyth misses the nimble light..

As an action,  the lights both become invisible

Can anyone see invisible?  Let me know if anyone is planning on crossing the stairs.

Checks and attacks are still viable.  If you act now, those close can probably guess where they were when they disappeared 

(@Wile_E_Coyote, Jaculyth has 'Blindsight', can she see/sense invisible creatures?)

Weneon will make attacks at disadvantage with her whip

Attack: 10 Damage: 4

Attack: 15 Damage: 4

Bonus action

Attack: 13 Damage: 4

Quote from InkedBee >>

(@Wile_E_Coyote, Jaculyth has 'Blindsight', can she see/sense invisible creatures?)

 Yes, she can.  They're still here.  You can sense them.

Also, Wen hits one once. (Throw enough punches...) and it pops back into view for all.

W1 Con save to remain invisible 1

Jaculyth flicks her tongue to taste the air, then slowly moves closer to Tabaqui and stares at him with 'snakey-eyes' (animal trance) in order to express her intention. If he allows her to use his leg as a twig, she will spin her tail around it and spring at least 10 feet towards the first Will o' Wisp.

Jaculi

Attack: Bite X Will o' Wisp

Attack: 20 Damage: 9 + Extra Piercing Damage 4  (Rolled with advantage)

A bit startled, Tabaqui will allow the druid to springboard off of him in her attack.  Meanwhile, he will draw his shortbow and fire at the visible wisp.

Attack: 27 Damage: 8 + Sneak attack damage (Weneon within 5 ft): 17 all nonmagical piercing

Marlyth gets a piece if the undead light, and Tabaqui finishes it off. 

The other one fails its save vs psychic damage and takes off up the stairs.

Con save to stay invisible 3