For this campaign in particular, the “Thoros abdicated and became a monk” ending is copied straight from his Wikipedia page, which currently says (untruthfully):
Thoros, weary after nearly quarter of a century of rule and warfare, abdicated in favor of his young son Roupen II, who was placed under the guardianship of Thoros’s father-in-law,[citation needed] the Regent Thomas.[1] After his abdication, Thoros became a monk.[citation needed]
Some Wikipedia editor had dutifully noted that the abdication claim has no source, but hadn’t verified that it’s a hoax.
The inspiration for this forgery comes from a later king of Armenian Cilicia Hetum II, who did abdicate twice, but in a way similar to Heian-era Japanese emperors, nominally fulfilling a devotional oath while exerting influence from inside a monastery.
The writer of this campaign story was either duped by Wikipedia forgery (like they had in the India DLC), or noticed the red flags, but willfully followed it through because they liked the thematic contrast of the spiritual Thoros to the mercenary Reynald.