The Codex of Xerrath

To further preserve it for posterity, the tooltip for this spell has read, in prior beta buids:

Permanently increases your spell power by 1%, reduces your critical strike chance by 1% and transforms the color of your Warlock fire spells mauve.

and

Permanently increases your haste by 1%, reduces your critical strike chance by 1% and transforms the color of your Warlock fire spells into a rainbow.

Also, summons Unicorns.

I find it interesting that nobody has mentioned the flavor text that was added to certain talents upon completing the quest to gain this effect. They're in the comments on some of the individual pages, but I figured it would help to compile them all.
[spell=110913] "All debts must be repaid...eventually." -Shinfel
[spell=108416] "Let someone else pay the price of your recklessness." -Zelfrax
[spell=108415] "The strongest of bonds is one which shares pain and pleasure equally." -Kanrethad
[spell=108482] "I will not be stopped - be it by Human, Dragon, or Titan." -Ritssyn

EDIT: This is now irrelevant, as these abilities either don't exist anymore, or no longer have the flavor text. A sad day for Warlocks everywhere.

To save it for posterity, because I'm sure parts of it will vanish soon -

The tooltip features this note:

Designer Note:
This side effects of this change can be unpredictable. Like inspiring premature forum posts about development spells before the questline is done.

It references a long-standing request for green fire from warlocks. And maybe some datamined information that was perhaps supported by some blue nudging. The last post by a blue had this to say:

So we’ve obtained some additional insight about what exactly lies behind the use of the word “hope” in regard to our efforts to achieve green fire for warlocks. Essentially, we want this updated information out so that we can better manage expectations, especially as the announcement created such a flurry of excitement.

Unfortunately, dear warlocks, those of you who retained a modicum of skepticism were right to, as it would seem that the chance of green fire for warlocks is even less as likely as the wording of the original information indicated.

Since spell effects are not as simple to change around as — for example — druid forms are, we need some additional technology implemented in order to allow the use of red or green fire to be a player choice and not a permanent change that is put in place for all warlocks.

Also, technology aside, we want to do the introduction of something like green fire in the right way. Implementing it in “a quest” doesn’t really explain our stance here. We want something as substantial as this change to be an epic accomplishment for you.

So though we have definitely heard your thoughts on the matter, and we’ve explained what we want to do from our side, green fire will not be available with the launch of Mists of Pandaria. And we haven’t a timeframe to commit to, or communicate about, at this stage.

It is with regret that we were unable to clarify these details more when we first mentioned our intent — “our hope” — and we wish we hadn’t caused such excitement and raised expectations for those that didn’t instantly take the news with an “I’ll believe it when I see it” pinch of salt. It seems, as many of us said at the time here and on Twitter and fansites, the proof of the (green fire) pudding really was in the eating.

eu.battle.net/wow/en/forum/topic/2986309301?page=4#73

Will it or won't it come? SPECULATION :3