The Codex of Xerrath - Is a passive ability that can be learned by completing the specific class quest chain starting from obtaining "Sealed Tome of the Lost Legion" https://www.wowhead.com/item=92426/s...he-lost-legion
By completing the whole quests & Black Temple's scenarios you suppose to unlock The Codex of Xerrath , which turns few of the Destrucation major spells to green-fire (Including your Felsteed & Dreadsteed).
The quest-chain is completable until you reach to Black Temple , at stage 4 , where you have to cross the Hall of Lost Souls which is full with invisible traps.
By using the Warlock ability "Eye of Kilorog" you were suppose to be able to see all traps clear as daylight but you cannot , traps are also not scripted at all.
https://www.youtube.com/watch?v=HdCtd0lXL6k&t=201s - Link to a guide from Youtube about the whole quest-chain.
Please give it some majority in your priority list , many Warlocks really want this , speaking from overall perspective , we've been farming for days to obtain this tome , which unlock one of the coolest if not the best feature that ever happened to Warlocks in this game.