Nick289.
Back when I was like, seven or eight, I had a cousin who introduced me to runescape. His name was Nick, and I didn't want to use my real name, so I sort of used his without his permission.
The 289 was just a bunch of random numbers that didn't mean anything. I used that name for a while until I sort of adopted DeathWard from his brother, Edward. He stopped playing runescape about a year after I was using Nick289, so he gave me his old account.
So yeah. First online nickname I used was Nick289, then "inherited" DeathWard from my cousin.