OLD NAME
My user used to be Nightbeast70 from 2006-2011. That name was inspired from Raven from Teen Titans, a show I used to love back then. I used a Dungeons and Dragons name Generator to play with words related to darkness/shadow powers that Raven had and nightbeast came out from there. When I typed the user in runescape, it was already taken and suggested something along lines of "Why not nightbeast70?" (Some random number). So I thought "eh, whatever" and took it.
CURRENT NAME
When I came back to Runescape in 2016, the interface prompted that I could change my user. I wanted a name without numbers and wanted one that sounds "Knighty" and "Vigilante-ish" at the same time. Gallant, Valiant, Bladed and valor were words that came into mind. Then randomly while playing I saw the doomsayer in lumbridge and thought "hm, doom sounds cool". Playing with words I got "Doom Gallant" "Knight Doom" "Bladed Doom" "Blade of Valor" etc. Eventually, I liked "Doom Valiant" best.
Later on I found out my name is similar to the name of the videogame: DOOM II: Valiant (a modded version of the 1994 original DOOM II game). That was not my intention, but all the more reason it made the name sound more awesome