Withers

The Withers item teaches you how to summon and dismiss Withers, a treant companion that follows you around, meaning it is a non-combat pet and will not aid you in battle. This pet is similar to the pet from the Argent Tournament.

Withers starts off as a rather large companion then will eventually wither down to a small size over roughly 25 minutes. If Withers comes in contact with a deep enough water source he grows to full size again and does the following emote in chat.

emote

Source
The Horde can buy this from Apothecary Furrows in Darkshore.

The Alliance have to complete the following quest chain in Darkshore, starting with:


 * 1a.)
 * 1b.)


 * 2a.)
 * 2b.)


 * 3a.)
 * 3b.)
 * 3c.)


 * 4a.)
 * 4b.)
 * 4c.)
 * 4d.)


 * 5a.)
 * 5b.)
 * 5c.)


 * 6a.)
 * 6c.)


 * 7a.)
 * 7b.)
 * 7c.)


 * 8a.) (can be completed alongside Swift Response)
 * 8b.) (can be completed alongside Swift Response)
 * 8c.) (can be completed alongside Swift Response)


 * 9a.)
 * 9b.)
 * 9c.)


 * 10a.)
 * 10b.)
 * 10c.) (looted off of the objective for Ending the Threat)
 * 10c.) (looted off of the objective for Ending the Threat)


 * After all those are complete, Cerellean Whiteclaw offers you the final quest, which rewards you with Withers.

Two pictures comparing Withers to the height of a Night Elf.