This is a somewhat time-intensive achievement that requires you to find and read a series of chapters on arcane magic written by Archmage Ansirem Runeweaver scattered around Dalaran. The reward is the Kirin Tor Familiar, itself a reference to the familiar "pets" of the magi that can be seen in Warcraft III (although, for the sake of accuracy, I think all of the pets you'll see there are technically water elementals).
The catch? Dalaran being a place built and run on magic, you can't count on any of these books even existing at a given time. They spawn in set locations around the city at 3 to 4-hour intervals, but the books that spawn in these locations aren't even guaranteed to be the arcane magic tomes you need. Some of them are random books just for flavor, but you will eventually get lucky and find the various chapters on -- among others -- Necromancy, Divination, and Illusion. Apparently this is Dalaran's less-convenient but more eccentric version of a mobile library.
Of most interest from a lore perspective are the Necromancy and Abjuration chapters. In the former is a reference to Kel'Thuzad's "current" status as a lich, and Abjuration narrows it down further with an approving-to-neutral reference to Kael'thas that (unless Runeweaver is simply glossing over a lot of history here) would seem to place the book's authorship in a nebulous time-frame around the midpoint of Warcraft III, or at least well before the events of Burning Crusade. Conjuration also contains a sly in-joke for all those of us who have visited the Shade of Aran in Karazhan.
WarcraftPets.com has a great guide on how to do this achievement here that I highly recommend.
The Coin Master
I've written about the Dalaran fountain previously, and it's really nothing but an avalanche of material from a lore junkie's perspective. 53 major and minor lore figures wished on coins that were subsequently thrown into the fountain, which has an uncanny knack for remembering all of them. Now it's your job to fish 'em up, and in your quest to do so you'll run across what's on Thrall's mind these days, the darkest part of Fandral Staghelm's history, a perfectly-valid question from Archimonde, and -- my personal favorite -- Jaina Proudmoore's private wish.
Question for the lore junkies among us -- when do you think Jaina tossed that into the fountain? As I've observed previously, that coin could mean one of several very different things.
El's Extreme Anglin' has a great guide here on what to expect while working on this achievement.
The Diplomat (Alliance) or The Diplomat (Horde)
Your own faction is comprised of several disparate races who don't always see eye to eye with each other (both physically and figuratively), but to other, unfriendly races in-game you're all pretty much the same. The vast majority of NPCs you'll happen across in Azeroth, Outland, and Northrend are all willing to start at neutral with you, but the Timbermaw, Sporeggar, and Kurenai/Mag'har don't have much reason to trust you right off the bat. All three factions are pretty hard up, and to them you're just another would-be hero who's going to have to prove his/her intentions are honest. Doing so is a sort of low-level diplomacy on behalf of your faction that convinces them to extend the hand of friendship (and some...interesting tabard designs) your way. "If you are a representative of your people," the Timbermaw leader will (finally) tell you, "then your people are ones we would work to make peace with."
While there's no specific reward apart from the "Diplomat" title you'll receive for reaching exalted with all three factions, the in-game event that's triggered when you turn in the final quest for the Timbermaw has always heartened me, and I hope that Blizzard does more things like this in the future.
Atiesh, Greatstaff of the Guardian
I went back and forth over whether or not to include this, as it's no longer doable (and thus a Feat of Strength), but finally decided -- why not? Atiesh was the weapon of the last (...sort of) of Azeroth's Guardians, an otherwise unbroken line of caster-warriors who battled the creeping demonic influence in the world, and you can't deny that holding this thing would have been a supreme achievement (no pun intended) for a lore geek. Not only did you have to reassemble the staff from 40 different splinters held by denizens of the original (and extremely difficult) Naxxramas, but you also had to battle the Old God C'thun (one of the most legendarily difficult encounters of its own day, and considered impossible in its original incarnation) for its base. In the end, you had Medivh's own staff, and could portal anyone you liked to the original, mysterious Karazhan before it ever became a raid.
Now? With the disappearance of Naxx-40, Atiesh has also vanished. As much as this disappoints me (I think the Atiesh model is one of the most beautiful weapons in the game, and would kill to see it reappear somehow), it's not entirely inappropriate given the relentless march of Azeroth's history. People die. Things disappear. Legends fade, as Medivh himself observed.
Hindering Kil'jaeden (otherwise known as Sunwell Plateau)
As if this needs description, but you fight Kil'jaeden. He's one tier of Burning Legion management below the ultimate Big Baddie himself, Sargeras, who we probably won't see (if we ever see him) for several expansions. You fight Kil'jaeden.
OK, it's a shadow of the actual Kil'Jaeden, insofar as he's nowhere near his full power attempting to squeeze his way into Azeroth through the Sunwell, but still.This remains one of Blizzard's best and coolest raid encounters, with one knockout punch of a lore moment immediately afterwards. The Shattered Sun contingent shows up (you guys couldn't have helped earlier?), as does Velen, and the Sunwell -- long since defiled by Arthas -- is reignited in a burst of holy energy. You helped to right a wrong that was one of the driving forces of the Warcraft III storyline and the reason for the general malaise of the blood elves.
Anyone with an interest in Warcraft lore should really see this cutscene, and these days, the fight is considerably more accessible than it once was. Get some '80's together, read up on the fight, and hit it up -- you won't be disappointed.