At the end ofOur Dreams Divide Us Still, a crucial late game quest inAvowed, you will need to make a choice between whether to listen toMihala and Kostya. Neither of them put up a particularly strong case. Mihala has led her people to the brink of ruin by stubbornly doing nothing under the guise of upholding tradition, is rude to you, and her plan consists entirely of undoing Kostya’s plan. It’s not a great pitch.
On the other hand, Kostya betrays Mihala to you, then betrays you to Lodwyn, and his plan means destroying Solace Keep at the risk of killing everyone inside in a misguided attempt to quell the Dreamscourge which may not be that effective anyway. Thechoice is therefore a little bit about the narrative, butmostly about your rewards. In that case, it isbest to save the Keep.

What Happens If You Leave Kostya In The Lava Tubes?
There is a bit of a copout option here, whererather than helping Kostyayou can simplytell him you won’t stop him, and thenleave. In this outcome, you getno reward and Solace Keep is destroyed. That’s what you get for sitting on the fence - this is theworst possible outcome for all concernedand should therefore be avoided.
When you return to Mihala in his instance, herresponse implies Kostya is still out there, but the quest immediately after this is the point of no return. As far as we could tell, thishint is merely Mihala being wistful,anddoes not have any ramifications. All in all, you should avoid this option.

What Happens If You Destroy The Winch Gears?
This option is at least taking a side, butthe rewards are the same nothing: Kostya gives you nothing but a ‘thank you’ for your troubles, meaningin terms of pure gains, Mihala is the better option. You might prefer an Envoy willing to make bold choices, but don’t expect shiny trinkets for doing so.
Mihala gives the samewistful responseabout Kostya in this choice too.
What Happens If You Kill Kostya?
If youchoose to fight Kostya, the first consequence is, well, you have to fight Kostya. He hits hard but is slow, and the same goes for his two Wardens. You can retreat down the steps for a bit of range, butdarting in and out to hit him with melee strikesand dodge his attacks is best. Hedoesn’t have any special movesbesides swinging his hammer, he’s just bulky.
After killing Kostya and his two Wardens, there are two things to do. The first is toloot his body, which gets youCouncilor’s Garb light armour(stats below) andCouncilor’s Shoes (+15 max Essence, +2 Dexterity, +25% dodge distance). After this, you can technically destroy the winch and follow his plan anyway, though this is fairly pointless and should only be done if you are a nihilistic wildcard who likes to watch the world burn.

5%
70

-20
Maximum Essence
-10
The second thing to do is toraise the gates by interacting with the wheelat the front of the room. This will raise the sluice gates,saving the Keep. Once you leave, head back to the Keep andspeak to Mihala, who gives you theBastion of Solace shield, whose stats can be found below.
84
20
340
45
What Do Your Companions Think In Our Dreams Divide Us Still?
The two companions who feel particularly strongly here areMarius and Yatzli. Given this isMarius' home, hefavours saving the Keep, and will be curt and angry if it falls.Yatzli, on the other hand,feels Kostya’s bold plan was the best course of action, and wonders aloud if you made the right choice should you kill him.Giatta leans towards agreeing with Mariusafter her own city burned, whileKai sides with Yatzli, though is fairly noncommittal.
The long and the short of it here is that while there is some narrative discussion, itdoes not mechanically impact your relationship with any companion, and so their feelings probably shouldn’t be of concern. Sorry, Yatzli.