It looks like the only currency that is doubled is the initial amount that appears when you complete the skirmish. Any currency you receive from a chest isn't doubled. Here's what the promotion says:
Anytime you would get Seals or Underdark Currency, you will get twice the amount.
Underdark Currency can be obtained from Demonic Heroic Encounters, Throne of the Dwarven Gods, Prophecy of Madness, and Demogorgon.
If you aren't going to award double currency for chests, you should specify that rather than explicitly state "anytime".
Comments
I don't recall if those instances have chests that drop that stuff or not - if it drops from there and isnt doubled, thats an issue for sure.
2X Twisted Ichor would be fun though 10 runs for a artifact set...yes sir.
Cryptic CMs are not known for their precise announcements
In your currency tab, under the "campaign currencies" section, you can see Demonic Ichor and Faerzress.
Twisted Ichor is under "other currencies" section.
These are all underdark currencies.
However, only Demonic Ichor and Faerzress are considered to be "underdark campaign currencies".
If you move your mouse cursor to Demonic Ichor and Faerzress in the currency section, you can see the description within the box saying they are campaign currency.
If you move your mouse cursor to Twisted Ichor, you can see it is "Treasure".
They're listed under currency, but I can't see any affiliation on them, on Xbox btw...