User blog comment:NaiflidG/Relating to Alastair in "On the Head of a Pin" and "Caged Heat"/@comment-212.3.18.48-20141113113050/@comment-11331875-20141116055042

I don't know about anyone else, but you've certainly convinced me, anon! Alastair being resilient but not immune to Ruby's knife seems like the likeliest and most reasonable idea, as it all makes sense: he was described as such in The Essential Supernatural, his statements ("You're going to have to try a lot harder than that, son" when Sam did it, and "Looks like God is on my side today" when Cass did) are ambiguous but more support him implying "I can be killed by it, though," he survived numerous stabbings with the knife and yet Dean acted as though he could still kill him with it, and Alastair's equal / possible superior Lilith consistently behaved as though she thought it could kill her. It was mostly the cases of Azazel dying from being shot in the same spot with the Colt and Abaddon being immune but reacting more strongly to the knife that gave me reservations, but your argument for why Azazel was not resilient to the Colt while Alastair was to the knife makes sense. If I may take this into wild speculation, Abaddon being unkillable but not totally resilient to Ruby's knife could explain why she is still affected by holy water (something that, in a practical sense, hurts demons but doesn't do much beyond that -- although I am still fond of the idea of angel-blessed holy water and that nowadays the Winchesters have Castiel bless all their holy water to make it super-effective against demons, which could also justify Abaddon's reaction to it even if she were ordinarily immune / resilient) but not by exorcisms (something that Azazel describes as something that "destroys" demons, even though it doesn't actually kill them, sort of like how Sam and Adam were described as being "dead" when they were trapped in Lucifer's Cage even though they were technically still alive). She can't be killed by anything other than the First Blade, but that doesn't make her invincible against everything else that affects demons.

Now that I think Alastair could probably have been killed by Ruby's knife, I do think that Lilith could've, too. She was far more resilient to Sam's powers than Alastair, and it's possible that she just didn't know if it'd kill her or not (who knows if she and Ruby could contact each other during the mission for Ruby to tell her that she could probably survive it because Alastair did), that she just wasn't in the mood to be stabbed because she figured that it'd still hurt like crazy, that she was playing up her fear of it / Sam to play on Sam's desire to kill her and the confidence that he could, etc., but her every reaction to the knife gave the impression that she was afraid of it. The only thing I considered a pretty solid piece of evidence in favor of her being immune to it was the idea that Alastair was, but since he probably wasn't, I'm more open to the idea that the only demons actually immune to its killing power were the unkillable Knights. As you said, though, judging by her aforementioned resilience to Sam's powers, she might well have been extremely resilient to the knife and probably even moreso than Alastair was; killing her with it would have been even more difficult than killing him with it. Ruby's reaction in "No Rest for the Wicked" ("You wanna go after her with one little pigsticker?") could support this, though it, too, is admittedly a little too ambiguous to draw a real conclusion from. At any rate, I imagine that if Sam and Ruby had gone after Lilith with the knife alone, without Sam being pumped up on two bodies' worth of demon blood, that fight would suddenly be a lot less one-sided -- or maybe it'd turn one-sided in Lilith's favor. (She probably would've still let him kill her, but Sam might be suspicious if she just stood in place and let him stab her until he managed to kill her, so I imagine she'd probably have to make it a little more difficult, if only to keep up pretenses.)

With regards to your last point, The Essential Supernatural hinted that Alastair could've been the second-born demon. I don't know if that is exactly canonical, especially now with the introduction of Cain and the Knights into the mythology, but it hasn't yet been contradicted and Alastair is implied to be incredibly old (even for a demon) so it's sort of an idea you can fall back on. Although -- as stated elsewhere on this blog -- I am intrigued by the idea of demons of different power levels potentially having unique powers and vulnerabilities not necessarily shared between ranks or other demons, my default position is still that higher-ranking demons probably possess all or most of the powers of lesser ones in addition to their own, and can often have less vulnerabilities than their subordinates. I also think that -- probably with the exception of Cain -- the Knights of Hell were typically above crossroads demons but below the Generals of Hell like Azazel and thus below the demon chiefs of staff like Alastair and Lilith. (I may or may not get around to making another blog explaining why I think the Knights are below Azazel. I ended up writing a lot about my theories on the demonic hierarchy and the behind-the-scenes interactions and motivations of different demons, when I was supposed to be doing actual work, so. Yeah. We'll see.) Although the Knights retain the unique quality of being unkillable to anything except the First Blade, Alastair, Azazel, Lilith, and any other higher-ups would therefore all possess many / most / all of the Knights' other powers while being even more powerful and resilient than them.