I took a look at both your Mannaroth and your Assault logs, as you seem to have only tanked Archie on H.
Mannaroth:
* Your spamming way too many festering strikes, this will drastically drop your dps as you could have used scourge strikes in there place.
* You only used Unholy Blight 3 times, possibly why you spammed so many festering strikes?
* Dark Transformation uptime was decent, could still get a bit more, make sure you store some RP away for it.
* As you mention, the class trinket is kinda meh at best on most fights. EDH is also not the best choice for Unholy, with UeH/Vial usually being our go to trinkets.
* Probably use multistrike food instead.
* Possibly could have gotten some more soul reapers, but it's hard to tell.
Hellfire Assault:
* Cast defile once. Big no-no, also it appears that you are having adds up for most of the fight with little downtime, so may want to take necrotic plague.
* Your dark transformation uptime is quite low.
* Looks like you had to run boxes, that will hurt your dps quite a bit.
* Still way more festering strikes than you should of had, also, it seems you plague striked almost every mob, would have been better to spread your diseases with a blood boil.
* Can get some nice AMS soaks on this fight, take advantage of them.
I do want to note one thing though, you might still be dong better than your other guilds dk, as for some reason on Mannaroth he chose to spam icy touch like it was going out of style, that's pretty bad.