Well, Mersadeon because... back in the day of teenagedom, my username was Adeon. That was the name of a car in some weird micro-machines-esque game (don't remember the name), and it was my favourite car in that game. Also, it looks nice and halfway pronounceable. But in the end, I had to realise that it isn't unique and thus is at times already taken, and I personally HATE going with "Adeon[number]".
Mersadion is the name of a general in "The Orks". I kinda liked the guy - he only has a few lines and doesn't appear often, he and his army were sold to the bad guy and so he has to work for her, being blamed for everything that goes wrong. I liked the way he always stayed reasonable. So I combined his name with my former preferred username and got Mersadeon.
It's handy. It is pronounceable, can be abbreviated easily and in two different ways, and people who knew me under my former username still have that vibe of familiarity when they say it.
EDIT: Also, should I ever need a pen name, I would absolutely go for Ratatöskr.