Note: Before I get to the bulk of the post, let me say that I couldn't think of a better place to put this. If someone thinks that it can fit better somewhere else, feel free to suggest.
Well, I've seen plenty of MOC topics, but absolutely no topic that are for one's MOC's backstory(s)*, thus, this topic was born.
Now, since I like to lead by example, I shall present to you the (rather rough) backstory for one of my better MOCs (and one of my few non-BIONICLE ones), Marendex.
Created by the most well-known robotics engineer on Silac, the T-17 Military Attack/Reconnaissance Android Model-X, was to be the front-most fighting robot in the Silacian Military. Unfortunately, a lightning strike fried the inhibitor chip and caused T-17 to go on a rampage, killing its creator and all the military officers at it's unveiling.
It began to go by the name "Marendex" which was both a combination of letters from its original designation, and a portmanteau of "Marel" and "N'Dex", the twin gods of destruction in Silacian culture.
It acquired its Nuclear Broadsword from a fighting master on a jungle planet, and began using it as his signature weapon.
10,000 years after its creation, its fusion reactor began to die, and would go out in a small supernova if not removed and replaced. It was removed and replaced by a less efficient fission reactor, which could not run for an eternity like its old one.
It began a job as a bounty hunter, gathering the cash to fund its need for nuclear fuel.
* watches topic get no traffic and fade into obscurity without a single post *
* Footnote: With the possible exception of RP character topics.