DM: The room the focus was in isn't very big. I'd say having 5 people standing around in it would be crowded so I'm going to assume you guys are laying the ambush in the tunnel itself rather than the room since the tunnel also has one entrance. With that out of the way, everyone roll Perception vs DC: 19

15

Devarus' Perception: 19.

Perception: 9.

Perception: 17

Rolling for Val: 17

At first all is quiet as the party waits for the creature to show up. There is no response. It seems that the creature either didn't hear it or choose not to investigate? That's when Deva spotted what looked like a slight shimmer in the air. It's invisible and heading straight for the party.

Only Deva knows the position of the creature as it moves but it is still invisible. The others don't even see the shimmer. Deva can attack as a reaction but others can't.

Party's turn.

Reaction: Taking the Shot!.
To Hit: 11 For: 7 non-magical piercing damage.
If hit, using Goading Maneuver, inflicting 3 extra damage. Wisdom Save[DC 15] or it has disadvantage on all attacks not on Devarus..

'Got you in my sights you fowl thing!' Devarus thought as he snapped Scattershot into position and loosed the bolts with a silent prayer its namesake prove more a blessing in at least hitting the invisible creature. If so, he barks out a curt, "Its you or me this time, Cretin. Don't think you'll slip the noose so easily!", goading him both in words and intimidating presence.

Dev's Turn:

"Its right where I fired my bolts! Rush it before it has a chance to slip away!" Devarus yelled as another set of bolts are shifted and locked in place. "Miss Elsara. Brace yourself. It likely will try to single you out again." He warns, sending in his words some of his ki to infuse her body. Though the effect would be weak. Even so, that doesn't stop him from firing another spray of bolts in the creatures direction while others engaged -- tried to engage in closer combat. With each volley unleashed, whether subconsciously or not, Devarus infuses a bit of himself in them to either unnerve the opponent or encourage another to advance under the covering fire.

Bonus Action: Rally Maneuver on Elsara, granting 2 Temp Hp.

Action: Attack w/ Scattershot.
#1To Hit: 21 For: 5 non-magical piercing damage.
#2 To Hit: 7 For: 7 non-magical piercing damage.

If #1 Hits, using Goading Maneuver, inflicting 4 extra damage. Wisdom Save[DC 15] or it has disadvantage on all attacks not on Devarus. (if previous Goading attempt did not work or was triggered)  or Maneuvering Manuever if already in affect, granting Runt movement as a reaction.
If #2 Hits, using Distracting Maneuver, inflicting 2 extra damage. Next attack on it by an ally is at advantage.

(To DM: Ammo deducted.)

With loud bangs. Deva fires a 3-volley barrage. Two clusters of darts from the heavy weapon stick to the wall behind the creature as it moves and swerves around the volleys. However, the third shot hits it directly with full force. ((Attack #1 hit. others missed))

Goading Maneuver WIS Save: 18

It hisses at Deva though it is still invisible.

Runt, Elsara, Gendan and Val's turn.

"Right!" Elsara hold her blade up, flourishing it as it flickers, pulling out from it are three shadows that split out from her, turning into three identical Elsaras.

Bonus Action: Bladesong
Action: Mirror Image

Elsara activates bladesong and creates copies to confuse the creature.

Runt, Gendan and Val's turn.

(Does the hiss give enough for us to attack at disadvantage?)

You can attack at disadvantage, just not using the reaction to fire ready actions is all.

Gendan fires two beams of Eldritch Blast at the location of the hiss.

Attack 13 for Unable to parse dice roll. force damage.

Attack 11 for 8 force damage.

The two beams fly toward the faint, almost unnoticeable shimmer. The water splashes suddenly as the beams strike the wall. The creature dodged.

Runt and Val's turn.

Runt will throw a Javalin in the direction everyone else is firing

Attack: 8 Damage: 6

Runt throws her spear with the force of a ballista. It seems to get stuck against nothing, seemingly floating in the air with the tip having vanished. The ensuing howl from the creature is a clear indication of a successful hit.

Thunder Wave: 10 Thunder. CON Save: 14 (DC15)
BA- Wild shape into Giant Octopus

Creature's turn

Grim Gash Recharge: 5 -recharging-
Death Strike Recharge: 3 -recharging-

Creature moves around using his invisibility to avoid AoO and goes for Elsara.

Hit:21 (Adv: 11) (Mirror image roll if needed: 13) Dmg: 8 Slashing
Hit: 26 (Mirror image roll if needed: 8) Dmg: 7 Slashing
Hit: 26 (Mirror image roll if needed: 5) Dmg: 11 Slashing

Two duplicates poof out of existence.

Forgot: Plunge into darkness recharge: 5

Lair Action: Plunge into darkness - Room becomes covered in magical darkness.

Val quickly moves up to the creature and releases a sonic wave. The creature ducks close to the ground, avoid most of the impact. Val quickly transforms into a giant octopus to tangle with the creature on more even size classes. The creature has other plans as it then moves like a spider and becomes visible right in front of Elsara before lunging at her head with the sickles. Two the Elsara images are struck and dissipate. The creature giggles and says, "You've destroyed one of my foci. That wasn't nice. That was a gift don't you know? No more mister nice Farm Boy." His image seems to become quickly obscured as the lighting in the room drops to near nothingness. The inky blackness returns. Strangely though, the room which had the hex focus remains unaffected by this inky blackness. It seems like this ability stems from a magically induced control over the house itself and the darkness is limited by the borders of the room the ability is used in.

Party's turn.

DM advice: If anyone has the Light spell or anything of that nature, consider using it.

(Is Deva’s Dancing Lights cantrip our only light spell? Anyone have a Driftglobe?)

(I'm pretty sure no one has a drift globe. Dancing Light would work I guess)