Getting the Felblood Portrait in World of Warcraft requires defeating the boss, Gul’dan, in the Nighthold raid on Mythic difficulty. Once Gul’dan is defeated, there is a chance that the Felblood Portrait will drop as loot. To increase the chances of getting the Felblood Portrait, players can do the following:
Join a raid group that is specifically farming for the Felblood Portrait. Coordinate and team up with other players that have the same goal to increase the likelihood of getting the portrait.