Hitting the level cap in WoW can be very overwhelming. The entire leveling process is very linear; go somewhere, do some quests, move onto the next place, and repeat. Once you reach the endgame, you have a number of choices of what to do next. Most of the options can be boiled down to two broad fields. You can concentrate on a lot of player-vs.-player scenarios like arenas and battleground, or you can get into endgame dungeons and large raids. At the time, raids were tuned in difficulty to call for 40 players, and could only be cleared once a week. The items that were obtained were much more powerful than those found in regular dungeons or from questing. When WoW was first released, there were two endgame raids available, the one-boss encounter in Onyxia's Lair, and the cavernous Molten Core.
Onyxia's Lair is the payoff to the storyline of the devious dragon who masqueraded as an advisor to the Stormwind King's son. After you unmask her, she flees the scene, and you follow her to her lair in Duskwallow Marsh and chop her head off. One boss, and that was it.
Molten Core is an immense volcanic cavern that houses the elemental Ragnaros and all of his minions.
Each section of Molten Core housed a burning rune that groups had to extinguish in order for the penultimate boss to spawn. The only problem is that the runes were guarded by each section's boss.
After all of the runes were extinguished, your group took on Majordomo Executus, a large salamander-like creature who had the power to communicate with Ragnaros. Groups didn't have to kill him; just take care of his eight guardians, and when they were defeated, Executus surrenders and grudgingly consents to summon Ragnaros, although he warns you, it will mean your doom.
Ragnaros is more angry at Executus, and promptly kills him. He then turns to you, and if your group is successful, you get a nice photo opportunity with the only thing he leaves behind...his molten hammer.
Blackwing Lair is the next step up in raid difficulty, and was implemented roughly 9 months after WoW's release. In this raid, you and your teammates storm the stronghold of Onyxia's brother, Nefarian.
The first room is unique in that you actually need to temporarily protect the boss that you will eventually kill, named Razorgore. You need to use his strength to destroy all of the Black Dragonflight eggs that he is guarding, so one of your raid members needs to use a mind control device to manipulate Razorgore, while the rest protect him from the never-ending wave of orcs that storm in and try to kill him. After you have him destroy the last egg, the mind control fails, and you are free to kill him. If he dies before all of the eggs are destroyed, a massive explosion will kill everyone in your raid.
In the next room, you find Nefarian in the process of subduing your friend from Blackrock Spire, the red dragon Vaelastrasz.
Nefarian has weakened him severely, and has corrupted him to the point where he will turn hostile against you. With his last bit of free will, Vaelastrasz casts a powerful spell on your raid that essentially gives you unlimited mana, rage or energy, which is important because this fight is a race to destroy the dragon before the spell wears off. His normal strength is imposing as it is, but he also singles out random raid members with a spell called Burning Adrenaline, which increases their damage by 100%, but kills them after a short duration.
After a number of other fights against various drakkanoids and drakes, and Nefarian's pet, Chromaggus...You come face to face with Nefarian himself, still in his human disguise.
Eventually, he sheds the costume and shows himself in his true form.
6 months after Blackwing Lair was released, the Ahn'Qiraj raid debuted. This raid was unique, because rather than have each person who wanted to experience it go through a series of quests to get attuned, the entire population on a realm had to participate in a war effort. This involved characters of all levels crafting food, bandages, armor, and other items, and turning them into members of the war effort. Once quotas had been reached on all items, an event began in the desert zone of Silithus.
The bug invasions in Tanaris and Un'Goro Crater, among other places, were in fact warning signs that an Old God buried deep in the Silithus desert had awoken and started causing trouble. Once the war effort is completed, there is a gate-opening event at the gates of Ahn'Qiraj, and raids could begin exploring the new content.
Most of the bosses in Ahn'Qiraj are, or resemble, giant bugs, like the raid's first, Prophet Skeram...or a giant wasp named Huhuran.
One of the most challenging boss fights was the Twin Emperors encounter, which involved two massive fighters who shared a health pool, and would heal each other if they got too close to each other.The raid's final boss is the Old God, C'thun, who first appears as only a massive eye, capable of doing massive damage.
Once you deal enough damage to the eye, C'thun reveals his true form, and in order to defeat him, some raid members must attack him from inside his stomach.