The Bone Rattler

is a suffix title given for completing the Achievement: Boneafied Threat.
 * The associated achievement can only be completed during the annual Nights of the Dead event.

Setting Titles
To set suffix and/or prefix titles after you earn them:
 * 1) Press C to open the Character sheet
 * 2) Click on the Options tab
 * 3) Locate the drop down to see all of the titles you have earned and select the title you want to display.
 * You can see the title displayed with your character's name at any time zooming out and by pressing F1 to target your own character.