Time for something more recent rather than old. This is something I did back march of this year, for a Discord server contest. It was about making a new biome for Terraria's underworld area. Me and a friend worked together on a concept for a flooded section, with boiling hot water springs and more "fresh" colors for such a red hot area.
The presentation advantage from making a background along with the most unique idea from all submissions made us the first place winners! Although, I can't show the items, enemies, armors and etc. we made to go along with it, since those were made collectively. I no longer remember how much was done by only myself, and even if I did, the interesting things were mostly made by my friend, and I don't wish to take credit for something he made, for the most part.
I did do the BG fully by myself, though. It took quite a long time, and I did it in the last days remaining to submit, but I managed. You can definetly see where the rush took it's toll if you have the eye for it, but it's nicely composed, in my eyes. So all's good.