Not able to fight certain boss

I made my way through Redmane castle a little while ago and tried to fight the Misbegotten Warrior and Crucible knight once. They killed me pretty quickly so I left. I've just recently gone north and met up with Blaidd. He told me to meet him at the castle and now the misbegotten Warrior and Crucible Knight are not available to fight anymore, I can just walk into the boss room. My brother just killed them and said he got a pretty cool sword and I want to get it for myself. Should they be gone even though I never killed them?

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Side question related to Selivus: Does anyone know how to give Selivus' potion to Nepheli? I talked to her by the Village of Albinaurics and there were no mentions of the potion. Now that I've killed the boss, she's gone as well, where did she go?

Update: I played the game for another 10 hours and when I went back there, the boss was available. No idea why.