Some spoilers
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I met the bookseller in Umar hills. He wants me to get blood of a Silver dragon! I want to get his boss, Vellin Dahn I think, but I dont want to kill the good dragon to get him. The dragon owes me a favor I guess for returning her eggs, can I just ask her for some blood?
Is there an evil silver dragon I can kill?
Should I just kill Cole and search his body for the location of his boss?

Sherpa Doug...running a Paladin

SPOILER:
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There is no such thing as an evil silver dragon in the game, and you cannot ask her to give you her blood either. It is either you kill her or you don't. I am pretty sure you can use the CreateCreature code to get another silver dragon, I don't know the spawning code for Aldon however.
If you want to "somewhat" conclude the Tanner Quest, just go to Trademeet and do the two major quests, then you will get a chance to kill Rajik once and for all.

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