Completing this achievement awards the title, the Heartless. It is also part of the requirements of the Freemarch Conqueror Achievement, although the tears do not need to be looted in Freemarch.
You need to kill squirrels for 20 of their tears for this achievement. The tears are not soulbound, but you must loot the tears. You will not get credit if you trade, mail, or purchase from the auction house. They are an uncommon drop.