The party and Jannell travel the Omerian Ocean on the Flying Elbow back to dry land, to the relief of most in the party! Captain Andy bids you all farewell, "ye land lubbers!" He then sets sail back on the high seas. A several week travel occurs through the Fairknot region, to the city of Ugash.

Ugash is a small settlement on the edge of the DoD. The populous is mostly humans, hobgoblins, and half-orcs who travel the DoD looking for treasures, rare animals to hunt, and sometimes turn in bounties on dangerous people who try to hide out in the DoD. The Ugashians are peaceful and welcome outsiders. They are hospitable to the party for the short visit. They offer the adventurers a discounted rate for renting camels for their long journey into the DoD and Obsidian Plain. 2GP each player at the promise the camels will be returned. You all set off on camel back into the DoD, for another long journey through the dangerous desert. After 10 days of travel and a few battles with lava magmin, fire elementals, and barely dodging being noticed by an adult red dragon (thanks to Reggie's keen eye) you arrive at the Obsidian Plain. 

The environment here is made of all black glass and rock. Spikes of growth and huge pillars shoot up randomly, making for lots of hiding spots, and no vegetation is in sight. Thick clouds overhead that hang like a dark smoke obscures the desert sun. The pillars resemble black claws reaching for the grey sky, as it shines the black reflective stone. It already feels 20 degrees hotter here than in the desert. This place is like hell on earth You all move slowly through the jagged terrain carefully. After a few more days, Reggie...you notice something lying on the ground about 150 feet in front of the party. You halt everyone and dismount your camels. A closer look shows about 6 of the Smoking Mirror cultists with shards of obsidian sticking out of their bodies as if a obsidian porcupine exploded next to them. You can tell they died recently, within the last few hours. You all look around and do not notice anymore Smoking Mirror cultists nearby. As when Reggie noticed the dragon perched on a rock in the desert, Reggie you have a feeling that something isn't right...that something is nearby the dead cultist. After taking a closer look at the obsidian shards sticking out of the ground behind the cultists, you see that one of the shards are shifting slightly. The shifting is in the form of a large humanoid. You think this might be a creature made of obsidian, hidden from most sight. 

What do you all do?

Gerry will stay back, not wanting his . . .less than stellar stealth skills to alert whatever it is.

Reggie would like to make some type of check trying to see if he knows what it is, nature (+0) or history (+3) check? 6

If Reggie gets any kind of felling that this thing is not going to be friendly he is going to start out by shooting it with his heavy crossbow.

Attack: 23 Damage: 8 auto crit if it hits +8=20

Sneak attack 16 X2= 38

Also if I understand things correctly, I now have crossbow expert, which means I do not have to use my bonus action to reload, and I can now instead shoot again, if so.

Attack: 14 Damage: 6

Did not show up.

(Crits double the dice, not the damage.  At least as far as I know.)

(Correct, double the dice. I posted in discord. Reggie will get massive damage regardless!)

This round is a surprise round. The creature did not see you all coming. 

Combat has started. Initiative rolls:

Ardeth: 19
Benagol: 4
Reggie: 6
Gerry: 9
Rubin: 7

Creature: 19

Surprise round initiative:

Benagol
Creature
Reggie
Rubin
Gerry
Ardeth

Benagol stands back as he has seen this look in Reggie's eye before. He holds his action in case the creature approaches. 

The creature is unaware of the party and is blended in with the terrain around it, but Reggie is able to detect it's presence. 

Reggie see's the creature and it looks similar to the fire elemental you fought in the desert, but it is made of the same obsidian material that is all around you. Reggie puts together that the cultists were killed by this creatures and fires his crossbow, activating his assassinate feature (damage to come)! 

Rubin, you're turn on the surprise round.

Rubin fires into the creature...

Attack: 6 Damage: 20

Attack: 20 Damage: 16

Bonus Action Attack: 20 Damage: 16

Dread Ambusher Attack: 17 Damage: 19 plus dread Damage: 3

Gerry will dash forward towards the creature.

(I'm assuming I'm not close enough for anything else, since you said it was about 150 feet away.)

Rolling Reggies critical dice (not doing double the critical dice roll). Cross bow damage: 5 Sneak attack bonus: 10

Rubin, two of your bolts bounce off the creature...not piecing it's natural armor. The 20 attacks hit (AC19).

Ardeth, you're up for the surprise round. The creature is being chipped away by Reggie's and Rubin's bolts...but is still standing. 

Ardeth will take the Dodge and prepare their mace. 

Benagol, seeing the distance to reach the creature moves up 30 feet and takes the Dodge, standing behind one of the obsidian shards sticking out of the ground. 

The creatures trudges forward a bit and looks like a walking obsidian shard, it's arms are club-like jagged obsidian. It's whole made has jagged obsidian stick out and it's hardened body is shiny with a red substance, it looks like dried blood. All of the sudden, the creature dives into the obsidian it stands on....and it disappears. The entry point and the surround ground does not look to be disturbed. 

Reggie, you're up! 

Reggie will ready an action, once the creature comes back he will attack with his heavy cross bow.

Attack: 15 Damage: 10

(If I have advantage 11 if I have sneak attack 15

(I hope I have sneak attack with that may 20!!!)

Rubin, Gerry, Ardeth. You're up!

(Can I get in melee range without dashing?)

You can get about halfway to where the creature was before it burrowed, without dashing. Currently, the creature is not on the surface.