Hello, I started the game and followed the path to Iorveth's and than in Act 2 , there is a mission about a succubus and its killing people around the town. But in order to not fail the quest " Where is Triss Merigold" I must go and meet the succubus because in the story timeline , quests like royal blood and rescuing Saskia rushes forward and if you happen to delay , it failes some of the quests. And Even though I took the crystal from the harpy nest , Philippa does not seem to have any option( no link in talk section ) about the crystal and it won't let me continue the quest to heal Saskia and go to harpy lair ? Does anything effect to make her accept the item or shoulld I begin from an ealier point in the game ?

And is there a way to improve the inventory stockage ? I found some item that makes it 255, could it be more advanced than that ?

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Do you mean the succubus of the "Flickering Heart" sidequest? If so: Why don't you finish this sidequest before continuing the mainquest? Saskia will not die (in chapter 2) whatever you will do. Promised. Did you talk to Cecil Burdon, who sends Geralt to a place in the north of Vergen? And has Geralt found a crystal there? Sell 200 of these items and you will be fine, whatever it is.

In the prologue there is a possibility to get the "Strong Back" perk, that depends on your choices in the game, though, and is missable (it'll up your storage to 300). Since you're in act 2 now, that possibility is gone.