I defeated him last night and he isnt in Sinfall. I think there is more to it than just completing the Sun King encounter.
This does not occur after defeating the Sun King's Salvation encounter. It occurs after clearing the entire raid on any difficulty or completing the Castle Nathria achievement (not sure which of the both).I defeated Sun King's Salvation (NM) on day 1 of the raid - No changeI defeated Sun King's Salvation (HC) later - No changeI defeated Sire Denathrius and all bosses but Council on NM - No changeThe change happened exactly when i defeated council on NM and completing the Castle Nathria Achievement/killing every boss on at least once on any difficulty