Tev actually got his back when he and the other death knights were still a part of the scourge. Since their main purpose is to conduct necromantic energy around his body, the tattoos and runes are used to connect him to his armor, giving him the ability to channel energy into it, much like he can his runeblade. 

He’s not really bound to a single set of armor, but he does prefer gear where he can take advantage of this. This also means getting everything in saronite, and having to go back to Acherus/Icecrown any time he wants to get it fixed.

As for the dick runes, If Tev thought they’d increase combat prowess, he totally got them.

Linking this so everyone can play along at home and because I’m at work and don’t have the original ref. (Also so Andrea goes ‘NICK NO’ in skype.)