This item is a quest reward from the climax  Admiral Taylor quest in the Spires of Arak storyline. Up until Patch 6.2, it was not eligible for transmogrification.
The flavor text about "Five Years of Distinguished Service" refers to Blizzard Entertainment awarding employees who have been with the company for five years an ornamental sword.
- Patch 6.2.0 (2015-06-23): Admiral Taylor's Greatsword is now eligible for transmogrification. If the item has been misplaced, the sword can be repurchased in-game. There is also a Horde equivalent in a different color that could be found somewhere in the Jungles of Tanaan as a treasure.
- Patch 6.0.2 (2014-10-14): Added.