While The Roach King is a pretty rare random encounter in TTW thing in TTW, there's not really much of a reward for finding him and ending his sorry existence, so I decided to correct it. 

Now, when you slay The Roach King, you will usurp his throne, and radroaches the world over will recognize you as their sovereign, thus becoming your loyal allies.

Edit: Turns out he's not random, I just suck at finding him. lol