[infael]

This is probably dumb, but I have the Ostmurk potion but cannot find Sile or the kayran. I tried to go back to where I found the mucus but that path seems blocked off.

The inn doesn't seem to have a top floor. Sile had said she was staying in the top floor of the inn.

[dragonbird]

Walk outside the inn into the square. Turn round and look at it. You'll see the top floor and how to get to it.

[MaskedReaper]

Go to the notice board and you'll see a stairwell on your right...go up the stairs and Sile room is the second door.
Talk to Sile and say 'you're ready' and this starts the Kayran battle mission.
You and Sile both walk towards the broken Bridge but you need to make your way down to where the ship wreck is to actually fight the kayran.
Just remember to prepare your drink potions before you get to the bottom otherwise a cutscene kicks-in just as you get there.

As soon as you start you almost get smacked by a tenticle if you don't move quick enough.
I once 'died in one hit' by a direct hit from a tenticle = 100% damage ...because I wasn't focusing what I was doing.
Learn't my lession that you don't stand around like a dunce.