Two comments. First, I absolutely hate Drath. She slows down *everything* so goddamn much. So you should just toss her in the vault for your own sanity :)
Second - Relentless is a terrible set for her IMO. If your goal is to get extra turns to lower the cooldowns on your A2 and A3, a much more useful set would be Reflex. Way higher chance of activating, and it's rare that you really need lower cooldown on both abilities simultaneously. Another very good choice for a set would be Frenzy - just faster turn meter fills.
But to be totally honest - I wouldn't go with either. Where are you using her? She's not a great arena champ at all, so I'm guessing you're using her for dungeons? If so, you probably just want to make her tanky so she never dies, and can be your failsafe reviver. But even there - I'm not terribly convinced she works there either.
So ... I guess all of this stream of conciousness boils down to - what are you using her for?