Is Morkeleb able to discern whether the moondial is indicating a certain phase of the moon? Or does he happen to know the current phase of the moon?

The moon has been waning recently. It was at 3/4 a few days ago, so the half-moon should be any night now, possibly even tonight. 

"i Guess we wait till night to see if this will give us answers." Jar suggests

Morkeleb is ok with waiting, as the immediate threat has been neutralized.

“But remember we need to hurry back to Bremen and show these heads to the lake monster, before the 7 days have passed.”

If the party chooses to wait for nightfall, they can benefit from a short rest. 

Morkeleb will use a hit die, healing 12 (maximum result, back to max hp 20)

(Anybody else that wants the short rest can add it to their sheet.)

The group takes a breather and waits for nightfall. In time, the moon comes out, and its light illuminates the giant dial you all stand on. As the light strikes the dial, one of the crystals glows more brightly, and you can hear the sound of stone scraping stone.

Inside the structure, the mystery door has opened. It contains a singular stone sarcophagus, similar to the one in Ravisin's room. The lid is sealed with both a heavy stone lid, and a pile of rocks stacked on top of it. The side of the sarcophagus has elvish runes etched into it.

For those who can read Elvish:

Jar takes a short rest (HD used)

Barnabus rests and gains 6 health.

Examining the container gives Barnabus a funny feeling.

"Har, anybody read that? All them rocks seems a little over done.)

(haha I just realized that my character can read elvish)

Morkeleb squints at the script and reads aloud for everyone’s benefit:

“Here struggles Sabnar, who refuses to stay dead.”

“Hmm, wonder what that means…” Morkeleb is inclined to not open the sarcophagus and find out.

Nox leans on the lid of the sarcophagus, "Anyone from these parts know anything about a Sabnar? If not any of you fine gentlemen, then perhaps somebody in town could recall something. Those bastards live long enough as it is, never mind one that refuses to stay dead. There's gotta be at least one person who knows something." He says.

After hearing what the inscription reads and While investigating the box, Barnabus put his ear on the sarcophagus. At the same time he uses a small hammer to tap on the stone. He listens and feels for any vibration.

 Perception: 20 (18+2)

Barnabua, you can tell that the sarcophagus is hollow, as expected. 

What’s less expected is that something inside the stone coffin taps on the side as well. With your perception roll, you can faintly hear a raspy, haggard voice from inside.

”Let… me… out… I can… reward you… for my freedom…”

Barnabus jumps back, his eyes wide.

"OH HELL! Somethings in there! It spoke to me! Said it would reward us for it's freedom! I've a mind to decline but if something living is trapped I feel I'm obliged to find out why. Anything else telling us what's inside?

Based on the statues by the entrance, as well as the general architecture of this place, whatever is in there likely is/was an elf.

The rest of the room is barren and dusty. You get the sense Ravisin didn’t spend much time in this room.

Morkeleb thinks it over a bit more and says, "Reward, eh? Hmm, how bad can one dead elf be? We're all rested up, I'm willing to see what the great Sabnar has to say."

Jar says "I say we open it and find out."

(Sounds like the party is in favor of opening it.)

You all take a few minutes to remove the rocks atop the sarcophagus, and with trepidation, lift off the heavy lid, dropping it on the ground. Slowly, a figure within the coffin sits up, perfectly straight. His mummified body is covered in very old wrappings, though enough have fallen off his head to reveal half a rotting face. His only other adornment is a chain necklace made of an obsidian black metal. His gaze regards each of you, sending chills down your spines. It feels as though time slows as it looks over each of you, until it finally speaks.

"Two hundred... thirty seven... years. No one comes here... Those I want vengeance against... Will be long dead now..."

The mummy considers for a moment.

"You have freed me. My word is... good. I will reward the four of you with... a choice. I can share... knowledge... or this damned necklace. It kept me alive... longer than I wanted to be. Will you take it... or shall I answer a question for you. Any question, so long as it doesn't use the word "should..." The mummy makes no effort to attack, but its scowl suggests it is almost reluctant to make good on the offer.

Morkeleb says, “Oh great Sabnar, your offer is very generous. Please allow us a moment to confer.”

Huddling up with his new friends, Morkeleb suggests they ignore the necklace as it’s apparently cursed, and opt instead for knowledge. But what to ask about?

Or anyway, that’s his vote on how to proceed.

Jar agrees with taking the knowledge option.