I think Christian and Barrons were successful in reaching that "something ancient" on Samhain.
Christian said he chickened out, but I think it was already too late.
Barrons did not chicken out because he was already familiar with the "something", therefore not afraid -- he already has it, keeps it suppressed with black & red tattoos of his own.
Christian's new black & red tats were supposed to protect him, but their efficacy in doing so is questionable, maybe because he chickened out, lost faith in runes and/or in himself, his Druid powers, and/or in Barrons. The tats were succeeding in keeping dormant what they had summoned until Mac fed him US flesh. Then, the physical manifestation of the presence in him was strengthened and became visible in the moving "tattoo" over his body, up his neck and around his throat. That was not a tattoo, IMO, but evidence of Draghar presence in him. In addition, he "was getting scary looking. His eyes were growing inhumanly brilliant, golden". (p.329)
In DH, the Druid Sect of the Draghar had winged serpent tattoos on their necks . I think these are similar to what Christian is now developing. Whether those Druids' tats were actual manifestations of possession or just tats Draghar wannabes is unknown. But I think they appear the same.
Barrons exhibited the same moving "tattoo" at his KMM interview, along with fading of his black & red tats. Maybe he has been lax in renewing his protective tats, or maybe the internal presence comes to the fore in times of stress, like adrenaline, and becomes active, more visible. (I don't have the CD, so am going by what I've read here concerning the interview. I apologize if I've misunderstood what was described. If so, please correct me so I don't go further astray.)
Ryodan's massive scarring along one side and up his neck (p.175) could indicate a removal (surgically or by other means -- expulsion?) of that presence. This would allow him to act on the 9's behalf in the human world, if he is no longer totally one of them, but still related "in a manner of speaking". Or maybe the scar remains from a failed attempt at excision.
QA is right to be worried about Christian. If this theory is correct, she has lost one of her Druids to the opposition -- the Draghar. She also has Darroc as opposition, so is justified in her worry about remaining top dog.
In summary, I think Christian is now entering a new stage. He is entering Barrons' camp, while maintaining MacKeltar roots, thereby straddling several factions:
1) Human/MacKeltar Druid
2) QA's Druid
3) Draghar Druid (if not Draghar, something commensurably powerful)
Christian has become a Major Player.