I disagree and I think while Corrupted Celestial Dragon is surely best used as combo card, he can't just be compared to Siphoner. Unlike Siphoner, he applies a global effect to the game. If your opponent by chance plays any healing ability the turn you deploy the dragon, that ability will be negated (he receives as much damage as he heals). Granted, the possibility of that happening is highly unlikely and with his high cost you won't see him played to counter a predicted healing, but as you said earlier about Vasir, you want to mention all the cards regardless of how good they are. Also, he can be combined with other cards from outside the list, like Field Engineer, to deal damage the turn he is deployed (however, cards like Solace's peace are much more effective, of course).
That being said, it is your list and I don't think that he is of such importance that he must necessarily be mentioned, just wanted to explain my train of thought on that.
I also would not add tome, because it has no immideate effect the turn it is played and leaves you with a couple of turns to react to it. It is not like you will make plans for the case that your opponent could possibly play tome of the dead this turn and adapt your strategy because you want to anticipate that a tome might be played this very turn, it is more like you see him play the tome and deal with it then (or maybe that is just me
And just because I saw it played recently and depending on your deck it is something you might want to be prepared to see, I would like to also suggest Grave Rob. Can be pretty sad if your Last Sleeper just disappears from game