When Benjamin Locke was hiding in the caves with the revolutionaries, they accidentally opened the Black Door allowing the Children of Leng to escape. Once Benjamin realized that regular steel was insufficient to hold the door closed, he crafted a padlock and key from Whispering Iron which proved sufficient to hold the door shut. Later, the lock was modified to be part of the door itself.
The key was only used to obtain Whispering Iron, and no infestation of the Children of Leng was recorded until the Tamers of the Tempest made their attempt to open the Black Door. After the events of that attempt, Rendell Locke vowed to ensure the Omega Key would stay hidden for the rest of his life.
When Dodge returned, he started hunting for the Omega Key, which had been concealed in the urn containing the cremated remains of Rendell. Afterwards, Tyler and Kinsey Locke rediscover the Omega Key. Tyler attempts to hide the Omega Key in his head using the Head Key, but Dodge (in Bode Locke's body) eventually gains possession of it.
The abilities of the Omega Key are currently unknown, other than it is the only key able to open the Black Door.
(damaged) ...another dead & the...dore open againe...the things turned to
irone...in our w...& I resolved to use them to crafte...hazzarded such a lock wood hold...long I worked in a fever'd...till finallie t'was cast &...oh how I feer'd! My har...in the gathr'd dark... straiked out &...held the dorr...beig'd God... Gonel...hold"
- This key and it's counterpart, the Alpha Key, were created in reverse order. As has been stated, the Omega Key was created first, and the Alpha Key was created last, about 237 years later.