I'm confused. Are you looking for help...
1. Writing fluff for Deathwatch-related units?
2. Formatting something for a boardgame codex?
3. Writing dialogue for Deathwatch units?
4. Writing plot for Deathwatch units?
5. Scripting/programming into a video game?
If #5, you may want to provide some more information. What's the engine you're using to create this? Is it part of its own game, or is it a mod for an existing game (such as Dawn of War)? My question probably seems obvious, but you mention scripting in one sentence, and then seem to focus on fluff-knowledge and general writing skills in another.