The Avernal Throne
At the throne of Avernus the Titan sits enthroned in fire. Only a full band walks back out.
A group raid opening at level 45, ending with Avernal Titan (level 50, ranged). Its antechamber lies in Gates of Avernus.
Rewards
A clear rolls the boss drop table several times over and can yield a raid gem for a weapon socket. See Raids for the party requirements, the daily lockout and the falloff on repeat clears.