There are several titles you can win just from killing certain types of mobs. While there are many titles only three are titles you can display.
Killer of Fay is just a step along the way to these. To attain "Hunter of (mob type)" you need 500 kills of that species. Killer of is at 100 and is not a displayable title. To get Slayer of you need 5000 kills. To get Destroyer of you need 10,000 kills at which point the game actually stops counting kills of that species. It isn't as hard as it might sound and most high level players get to Slayer of and Destroyer of Undead without trying.