The Believer is a quest to kill Suzerix, Adalwolf Elder. It was level 41, but as most quests below level 70 or so, was bumped up to make room for the Eclipse expansion quests in Celestial Vale. It is now level 46-51; a range of six levels, the narrowest of windows for completion
Suzerix was level 50, as the video link shows. He was actually leveled down to level 45. The effect on the actual stats is another matter, but it is safe to assume they were also reduced.
Suzerix shares a creature model with other bosses, such as Krixxix, and many regular monsters, such as the 'Primycete' Adalwolf Archons near Heaven's Tear
Sumor Camp Elder -334 827
Coin: 473, XP: 47,250, SP: 10,679