[X] You will ease into the conversation by explaining that for most of the Lords of Westeros that you've visited, defenses against the more insidious foes and threats, usually the most prescient and powerful, have been rather slim, so you've used your own wealth of experience dealing with cults and fiends and other foulness by generally cleaning up the immediate problems most are facing whom you have already visited. It just so happens that whatever ancient threat that had risen up wasn't at all lying dormant through Magic's slumber like most, it was that which threatened both House Blackwood and House Bracken for nigh uncountable generations.
-[X] You lead him along the threads of insight as you followed them, proving not only your competence but the care you placed into the matter, the sword, the Lady, chains of logic succinctly lain out and hopefully your cleverness and light touch endears you to them, if not the meat of the subject, most of what you tell him, if it had come to pass, do not paint a pretty picture for the depths of depravity and loathing Rickard Stark would sink to. You paint Lady Catelyn in the best light that you can, hoping to use clever conversational turns to render any ire he might feel away from Brackens and onto the monster which tried to engineer a horrid fate for both Lords' children, and ultimately the destruction of their lines. No more to blame than Hoster or really either of the High Lords for all that, as they had danced on unseen strings since the very day they were born.
--[X] You know this will spark anger, not necessarily at you, maybe rage at Rickard, though you had the idea to sacrifice him--after extracting as much useful information from him as possible--in order to both help empower the Raven Tree, but perhaps also make Hoster Blackwood a true Druid with a connection to the Old Gods. The important part is the distinction that the ancient feud, thousands of years of intricate, maddened schemes, even the very foundations of it such as a poisoned Raven Tree, can be lain at the feet at either all of them, or one of them who ultimately won't get to be the one who decides that the only legacy left behind of the grand tragedy shall be the ashes of the dead and forgotten.
---[X] Moreover, this may perhaps be the last chance for a clean break. What else could convince him, short of Jonos Bracken of all people being tentatively open to a true marriage and dynastic alliance, unshadowed by hidden barbs and last minute spite? In any other world but this one it would be impossible to extract such a huge change of heart were what we spoke here not both completely accurate, but more importantly carrying a more poignant truth:
----[X] While it might have been a Blackwood who lay at the heart of this madness, if we set aside other ancient sins, far and away from living memory as they are, who has the right to hold those living now, with responsibilities to to both their living heirs and their subjects, accountable for what might set aright those deeds, and not at the hand by some vengeful specter or distant liege haphazardly telling both sides to shake hands and get along, but those scions who today can make of the world a brighter and kinder place?
-----[X] Wearily, you close your account with the following: "Ultimately my Lord, I implore you to consider this, please. For years I have faced all kinds of horrors, prevented the worst atrocities from being committed on Westerosi soil, often times for people who wouldn't feel an ounce of gratitude for the efforts. It was my duty, yes, but it was also a thrill and a pleasure to stop monsters or alien minds from just having their way with us all. This tale? It's not my glory, for I shan't be sharing it far and wide, nor does it bring me pleasure that so much sorrow was woven together much like a serpent devouring its own tail. It's not a story simply of monsters and wraiths from the past, heroes stepping forth to stop them, or glory and accolades thus dispensed. It's a tale of men and women. No one will learn of it outside of these two Keeps and in the councils of Kings... but perhaps that is for the best? Anything else would just earn us the laughter of thirsting gods."