The Kayran: Ostmurk is a Witcher 2 Assassins of Kings Enhanced Edition quest that must be completed in order to advance through the games main story.
The Kayran: Ostmurk is the third part of the quest involving the huge monster that attacked Flotsam during the first act of the game.
As soon as you find the mucus in the swamp north of Lobinden, you will activate The Kayran: Ostmurk.
“The kayrans mucus was not poisonous, but also magical, a fact Triss Merigold confirmed beyond all doubt. An herb known as Ostmurk was necessary to prepare the antidote.
The plant, as is widely known, grows underground in very damp places. Geralt considered whether to search for such a place himself, or to ask Cedric for help.”
After Triss teleports out of the swamp, you must go back to Lobinden and seek Cedric, the elf you encountered during the previous quest. You will usually find him up on the platform, next to a tree, watching over Lobinden.
Talk to him and he will tell you that you must travel deep in the southern forests, to find some Ostmurk. This special herb grows in a cave, beneath the waterfall. After you kill the Drowners near the lake, enter the cave, through the waterfall.
Inside you will have to defeat many Nekkers, but if you completed The Nekker Contract you already know how to beat them.
Note: Make sure you fully explore all tunnels because there are some corpses that carry valuable items.
The location of the herb you are looking for is clearly marked on your map, if you track the quest, so you will have no problems in finding it.
When you approach the marked spot stay alert because you must defeat a Greater Rotfiend.
Combat Tip: If you manage to stun the Greater Rotfiend, use strong attacks, to kill the creature.
As soon as the Greater Rotfiend dies, retrieve the glowing ingredient on the caves wall and create the Mongoose Potion. You already have the formula.
Note: To create the potion, you have to meditate. Press RB and then A to access the meditation menu.
After you create the postion, return to Flotsam and visit the second floor of the inn. Talk to Sile and tell her you are ready to face the Kayran.
Sile will take you to the Kayrans lair. Descend the hill, following the path to the right. Count the ledges and after you jump off the second one, stop.
At this point you should drink the Mongoose Potion you have created. Make sure you combine it with two other potions that increase your resistance to poison.
After you consume the potions, continue to descend the hill and a cutscene starts showing Sile who calls the Kayran using one of her spells.
When the fight starts, you will see that some of Kayrans tentacles have some inflammations. These are the weak spots, but the boss moves the tentacles all the time. To stop tem, you must use your Yrden Sign.
Start with those on the right side. Using the Yrden Sign, place a trap on the ground then make sure Geralt stands near it. When the Kayrans tentacle begins to fall towards the trap, step out the area (roll around). Eventually, the tentacle will be immobilized.
Approach it and land some strong attacks, to cut it. Repeat this step on all tentacles on the right, then focus on those on the left side.
To avoid the Kayrans sweeping attacks, dodge left or right.
From time to time, the Kayran will grab you, and to escape, you have to mash the button that appears on your screen (usually A). When all tentacles have been destroyed, the boss will smash one of the structures above him.
When it falls, hurry up and climb it using the ramp on the left side of the area. Roll towards the monster and watch the cutscene showing Geralts final attack.
Next, talk to Sile and she will tell you to pick up your reward in Flotsam. At this point you will complete the quests involving the huge monster.
Note: Dont leave the area without looting the Kayrans corpse. The Kayrans Trophy is an epic item which increases Geralts vitality.
Return to Flotsam and talk to the merchant in the port for your reward. You can now focus on other side-quests or main quests featured in the game. One of these quests should be The Assassins of Kings.