Bart approaches the robed individuals. 

"Ho there. What business brings you out here?" 

The guards are startled by your approach, but quickly raise their shields and point their spears at you. The one smoking the pipe says in a whispery voice, "We could ask the same of you, friend. I am Jondus, and we are just part of a team investigating the wonders of this old 'hill'. What business do you have with us?"

"I am tracking a missing girl from the town. It appears she came this way. Have you seen her?" 

“Nope, haven’t seen her.”

"Then either you haven't been here long or she managed a bit of invisibility. I'll need to check within your cave." 

In the undergrowth, Cariadne readies her crossbow, just in case. 

Jondus says “Of course, come take a look around.  Nellis here, “ gesturing to the woman with the scimitar and a snakeskin tattoo on her arm, “will bring you in and show you around.”

(I never thought I'd make it this far... Lol)

Bart will head inside with Nellis. 

"So what is this place?" 

Can Cariadne see the tattoo? Does she recognise the snake?

Perception= 18

(I didn't think it would go this far either!)

Cariadne: You do see a tattoo on the woman's arm, but at this distance it isn't clear what the design is. You can hear most of the conversation, but something about their tone seems off and possibly threatening.

Bart: As you turn to follow Nellis, you get a very bad feeling about the situation.

(Passive Insight was used.)

Jenniwyn says, "Wait, he's going in? I don't like this..."

Bart stops. "I believe I will need to hear the truth from you. What did you do with the girl?" 

Cariadne knows, deep in her gullet, that this is going south, and fast.

She makes a quick birdcall noise to alert Bart, and pings a crossbow bolt at Jondus.

Attack: 14 Damage: 12 (if she is able to use sneak attack, plus 8 damage)

(and rolling for initiative - 17 )

Bart - Give me an initiative roll.

Cariadne, your crossbow bolt strikes Jondus in the chest, wounding him badly. (You are effectively hidden, so sneak attack applies to that roll.)

Jenniwyn casts Faerie Fire on Jondus and the two guards. The guard manage to avoid the light but Jondus is glowing, granting advantage to any further attacks on him.

Bart still has a round 1 action to go.

The enemies rolled poorly on initiative, so you will likely get a second round of actions after the surprise round is resolved.

Quote from Charwoman_Gene >>

Bart - Give me an initiative roll.

 Bart initiative 12 

Bart is up for Round 1, the enemies are surprised.

Then Cariadne, Bart, then Jenniwyn act before the enemies go in round 2.

Cariadne shoots again at Jondus.

Attack: 16 Damage: 5

(if Bart has already dispatched Jondus by the time she has her turn, she will aim at the next closest guard.)

Bart would swing at the guard right next to him,which I assume is Jondus

Attack: 21 Damage: 13 

Holy crap, if I can, I'd like to say I skewered her entirely, lifting her up off the ground like classic Sephoroth v Cloud move lol

Okay, There is Nellis (N on the map), the female cultist in leather armor and scimitar that was leading you into the tunnel, and not affected by the faerie fire. Your pronouns indicate she was your target, so I'm assuming that was the case, and you skewer her, even without the critical damage she's dead and impaled.

Jondus is J on the map, next to you, and the one calling the shots. The Guards are Sp on the map (spear) and are in chain shirts. 

Cariadne, after seeing Bart stab the person leading him into the cave, you place a well-aimed strike into Jondus' leg. He is looking badly wounded. (You had advantage so I rolled sneak attack for you.)

Bart is up for round 2, followed by Jenniwyn, then the enemies.  All attacks on Jondus have advantage.