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 Teldrassil Sproutling 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.) The Last Wave of Survivors
 * 1b.) Threat from the Water


 * 2a.) The Boon of the Seas
 * 2b.) Buzzbox 413 (quest)
 * --- No Accounting for Taste
 * --- Buzzbox 723 (quest)


 * 3a.) A Cure In The Dark
 * 3b.) The Corruption's Source
 * 3c.) A Troubling Prescription


 * 4a.) A Love Eternal
 * 4b.) A Lost Companion
 * 4c.) The Final Flame of Bashal'Aran
 * 4d.) Solace for the Highborne


 * 5a.) Unsavory Remedies
 * 5b.) Twice Removed
 * 5c.) Ritual Materials
 * --- The Ritual Bond


 * 6a.) Grimclaw's Return
 * 6c.) The Shatterspear Invaders


 * 7a.) Remnants of the Highborne
 * 7b.) Shatterspear Laborers
 * 7c.) Reason to Worry
 * --- Swift Response
 * --- One Bitter Wish


 * 8a.) Denying Manpower (can be completed alongside Swift Response)
 * 8b.) War Supplies (can be completed alongside Swift Response)
 * 8c.) Timely Arrival (can be completed alongside Swift Response)


 * 9a.) On the Brink (quest)
 * 9b.) Strategic Strikes
 * 9c.) The Looting of Althalaxx


 * 10a.) The Front Line
 * --- Ending the Threat
 * 10b.) The Ancients' Ire
 * 10c.) Disturbing Connections (looted off of the objective for Ending the Threat)


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

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