They say to mix all components and that includes relics.
So i believe you can choose any relic because the rules don't actually say that relics are specific to campaigns
since you're expected to combine all components of each expansion into your game (monster groups, classes, tokens, etc), it doesn't make sense that somehow relics are an exception to this.
Firstly, neither of these statements are true. If you look at the expansion rules (in Labyrinth of Ruin, for example) it says (emphasis as written in the rules):
The new Labyrinth of Ruin Lieutenant, Relic, Ally, and Ally Skill cards are used only when specified in a quest included in this expansion.
This is also repeated again in Expansion set up:
2. Set Aside Campaign Cards: Place the Lieutenant, Relic, Ally, and Ally Skill cards from this expansion aside. They are used only when playing the “Labyrinth of Ruin” campaign.
That said, Justin has clarified that the rules of that quest reward are an exception to the general rules. But using "It says to mix them all together" as a justification for a rules decision would be an incorrect method of coming to that decision, because it specifically says NOT to.
While I would probably allow for any Relic from the base set, it's still unclear if Justin intends the base set to be allowed as it's not technically an expansion. Personally, I think that if he meant it to be excluded, he would've said so, but it could be argued that he is just referring to additional relics from actual expansions. (But in that case, if playing The Shadow Rune campaign, then you couldn't take other Shadow Rune relics, which wouldn't make sense)
If you aren't playing the Shadow Rune campaign then the Shadow Rune relic will definitely qualify as an unclaimed relic.
Even under the assumption that you do include Relics from the core set, this would be completely pointless as Zachareth is the only Lieutenant than can wield The Shadow Rune, and, unless I'm mistaken, if you aren't playing The Shadow Rune campaign, Zachareth doesn't appear as a Lieutenant. (That said, it is possible that he could make a resurgence in another campaign, or in a Home Brew one, which would allow for it.) Even if you are using his Plot Deck, when you summon him, it is as an Agent, which is NOT a Lieutenant, and CAN'T wield Relics. If you are playing The Shadow Rune campaign, then The Shadow Rune Relic will have been claimed by that point, so it wouldn't count as an option.
Edited by griton, 05 February 2014 - 10:28 AM.