Given the explanation that he just heard Andivus will shout to the whole party.  “Destroy anything that’s out of place, it’s the work of devils!”  

Andivus points his trident and fires off an eldritch blast at the target. 2 added from bless for attack

Attack: 24 Damage: 7

He Then begins to close the distance between him and the creature.

As they hear the commotion outside of the ruined stables, Lia and Oswin try to analyze the circle. While Lia is unable to discern much despite her magical prowess.

Oswin, however, is able to recognize it as a summoning circle. Not to bring something into the world, but to bring a large portion of the forest into something. Whatever is in the middle will anchor it to wherever its destination is, as well as the circle's power source.

Outside, sensing an impending fight, Valentia prepares half of the group for what may come as she relays what it said to those present, answering Gark's question.

Suddenly, Andivo sends a streak of energy directly towards the frightful creature showing no mercy to such an abomination. It strikes it in the chest (7 force damage). He then charges forward.

Recovering from the sudden attack the creatures tone of voice gets darker.

In Infernal:

Roll initiative!

Creature torso: 3

Creature mount: 3

(Even though this thing has two initative rolls, it has only one HP pool)

Initiative: 23

If he can before combat starts, Oswin will use his spear to try and push whatever’s in the center of the circle out, otherwise I will figure something out when its his turn.

Gark is started by Andivo’s attack but readies for battle once he sees the horse demon charge. 

initiative 5 

(about how far is the horse from us?)

(@Ismenow: Not now since initiative has been called. You can certainly try on your turn, though. When you do, you'll need to pass a [DC15] Strength Save first. If successful, to push, I'll have you roll a d20 as if you were attacking it.)

(@happyandbob: It is 45 feet from the stable. Depending where you are, you may be closer or farther away. The map will come once initiative has been rolled)

Andivus initiative: 14

Lia initiative:  22

(Gark was doing a perimeter sweep so I'd imagine he's also about 45-50 ft from the stable, but he's probably also the same distance from the figure since Gark spotted him from a distance?)

(If Gark was that far, Valentia would have to be 30 ft. between Gark and the party before she cast Bless, and then she'd move inside the stable to be with the rest of the group. While standing in the back in relation to the dark figure.)

Initiative: 20

 Initiative order:

 Map

 Oswin, Lia, Gark and Andivus can take their turn.

Gark uses his bonus action to cast Zephyr Strike (movement to 60, no opportunity attacks)

He swiftly moves 30ft to take a swing at the torso with his long sword (with ZS advantage)

Attack: 23 Damage: 8  + bless 3 + ZS damage 1

After his attack, he will glide diagonally back toward the party at E15

Oswin will use his spear to try and push the focal object out of the circle, hoping to end whatever its doing.

11 (You said to roll a str save then said to roll as if it were an attack, so I rolled straight str, if its an attack, please add 2 for proficiency.)

(Edit: NM, either way looks like it failed.)

Oswin will turn to Lia, "I can't get it, but I think if we can get this thing out of the circle it'll stop the spell."

"I will give it a try..." Lia says.

Strength Save:  14

Dagger attack:  14

Favored by the Gods (Special)
Once per short rest, if you fail a saving throw or miss with an attack roll, you can roll 2d4 and add it to the total, possibly changing the outcome.

Bonus to attack: 6

Andivus moves closer as his eyes flash red momentarily, speak in a foreign language.  A swirl of black energy like black fog falling off the clings to it as he casts Hexblade’s curse,( on the head?) before he fires another eldritch blast

Attack: 6 Damage: 15

Gark closes the distance and throws one of his javelins. Seemingly assisted by the wind itself it strikes true, burying the head deep into one of its arms (9 piercing damage).

Following suit, Andivus attempts to send another blast of energy towards the creature, however he misjudged the amount of energy he put into it and it instead explodes with a loud POP (I don't know if you read the campaign-specific rules or not, but rolling a nat 1 in combat you will need to roll a DC10 "fail save." You can either roll now or on your next turn. Success avoids a -4 penalty to attack rolls for the next turn.)

Inside the stables, Oswin takes his spear and tries to push out the object acting as its focus. As he does, a persistent wave of energy starts pushing back on him from the circle itself. He is unable to push though and instead is shoved back 10 feet and knocked prone.

Lia tries her hand at the circle and is able to overpower the energy waves, and in one swift motion, swipes at the object. It is sent skating across the former floor and ends up through a hole by the door, settling just outside. As that happens, the magic circle starts fluctuating from blindingly bright to in-perceptively dim at a rapid rate, which slows down until it is no more.

Creature mount's turn

The abominable mount rears as it lets out a sound best described as a roaring whinny and rushes out of the more heavily forested area. Once in the middle of the party, all of the eyes of the creature turn purple and both let out a terrible sound, not only bombarding the group's ears, but also dazes them (Gark, Andius and Valentia, roll [DC14] Wisdom save vs. 9 thunder damage and stun). Their eyes fade back to red.

Valentia's turn next.

Lia ignores the horrible sound coming from outside and moves after the object quickly, wanting to make sure it does not get lost,or taken by whatever made that sound outside.  (Assuming she has not seen the strange creature as of yet.)

(@DragonDenn: You're still in initiative, is this going to be your next turn in the next round?)

Yep.

(Something to note. Lia has the effects of Bless, but neither of the rolls @DragonDenn made had the bonus +1d4. It seems it didn't matter because, apparently, both rolls succeded. Edit: The same technically goes for Andivus, but that also doesn't matter because he rolled a Nat 1.)

Valentia's Wisdom Save: 20

With a tranquil mind, Valentia overcomes the effect of the mount's scream. She gets closer to it (K13, it should be within 30ft. of them) and raises her hand to grab at the amulet around her neck. She uses her Channel Divinity to curse the creature on top of the mount with Path to the Grave. She then tells her companions. "Quick. The creature on top should be weaker for some time, if you can hit it with a strong attack, we should be able to take it down easier." And then, with the rest of her movement, she goes back to a place where she might be able to get some cover. (N12)

 Gark wisdom save: 21 + Bless 1