9. C'Thun - World Of Warcraft
Everything about C'Thun embodies World Of Warcraft's wild early years. Long teased, gaining access to dreaded Old God himself was a historic event.
It began with a server-wide event requiring the contribution of all players to grind and turn in materials. Then, a series of extremely difficult quests had to be completed to craft a unique item. At that point there was a ten-hour long war in which the Alliance and Horde had to work together to defeat an army of C'Thuns minions. And only THEN could you enter the 40-person raid that culminated in this fight.
So you can imagine the how anti-climactic it was when even the most elite player guilds in the game couldn't beat him.
On top of requiring lighting fast precision and coordination among 40 players, C'Thun also had a (bugged) attack that could knock players down repeatedly. And upon losing a certain amount of health, his strength would increase to a point where victory became impossible.
This went on for months until Blizzard finally released a patch nerfing C'Thun. Unfortunately, it also turned out to be a huge over-correction, as he was easily defeated the very same day. Today, only the most veteran players even remember C'Thun.