I don't know if I would call it 'hard' it just required some pretty specific class and legendary stacking in an era of endgame raiding where guilds were not ready to pull in an endless amount of of geared x or y class like they are today.
Mechanics were always easy it was just some really tight tuning.
Hi Sephurik
No, but I will give you that tanking the bloods was not the most fun tanking experience I've had.
Looking marvelous in velvet.
Sorry, but M'uru was killed 6 days after being available, the second sunwell gate opened april 29th, M'uru was killed may 4th by SK Gaming. Granted, 6 days is in the upper region, but definetly not hardest.
C'thun was bugged and literally impossible, the bugfixes came with nerfs, so we will never know how hard a pre nerf-c'thun would have been for that time.