Yes. You weapon is horrible. Seriously. Ti'tahk sounds cool, reads cool, but is such a terrible weapon. In all seriousness.
From a post a while ago...
An even funnier comment I saw...Comparing Nibelung to other ICC weapons and Ti'tahk to current ones, Nibelung was soooo much better. And Nibelung sucked.
But lets get into the good stuff, other gear and logs. I will focus gear talks on destro, your current spec. If you have from VPL from rag, try it out. Last I saw, it was better than Bottled Wishes. Also, if you still run Heroic FL, the heroic domo neck is BiS over the VP one. The haste cap for destro (by haste cap I mean the point that haste is less valued over mastery/crit, which one it is depends on the rest of your gear and should be simmed), is 2681. You are about 50 points over that, so some reforging could help that. Try out wowreforge.com. (future gear note, the normal tier shoulders/spine gloves combo is better than heroic morchok/tier gloves combo. So don't go oooo heroic shiny all over it if it drops)
Now for logs. I'm going to go back to your last Heroic Ultrax Kill first since if you're messing up there, you're messing up everywhere else. Your opening is slightly off. The optimal destro opener is something like: pre-pot > Pre-cast incinerate > combat > Soulburn:Soulfire > CoE (if needed) > Immolate > BoD > Conflagrate > Corruption > Shadowflame (if in melee range) > Incinerate x3 (under backdraft. 3 stacks). From here, it's based on priority. Conflagrate > Incinerate (under backdraft) > Chaos Bolt > Incinerate (no backdraft).
Now dots. Immolate is 91% uptime, your best one, but still needs to be more like 95%. Work on clipping it right before the last tick so that it is extended. As for corruption, need to make sure you're clipping it before the last tick as well, or even casting it. It'll be on for a while, fall off for a while of equal length, then back on for maybe a lil longer, back off for far too long, etc. Uptime was only 50% :x As for BoD, you're clipping it too much (I'm assuming you're clipping it too much instead of tracking procs to know when to clip it, based on the poor corruption uptime. Most locks who pay attention enough to procs with BoD, also make sure to keep dots up first). BoD is only clipped when Buffs present > Previous buffs when cast. When in doubt, don't clip. And put it back up asap.
Your shadowflame on CD use was great. A+ there.
Need lots of improvement to your ISF (Improved Soul Fire) buff uptime, only 64%, and it's one of those "we really want this up 100% of the time" buffs. It appears that you're using your soulburn on CD, which is great and helps keep that buff up. On ultrax, the timing works out that soulburn will be ready after every hour of twilight. So if the buff is about to fall off, but you need to fade out for hour of twilight, it's like "hey, no worries mon, here's a free soulfire and a buff!" Even if you don't get soulburn, or an imp proc, you need to keep it up, which means hardcasting it. Have some sort of tracking on it, when it reaches 2-3s left, that's when you need to start recasting a hardcast soulfire. If you just hardcasted a new soul fire and the imp procs a new instant soul fire, don't use it immediately, wait a couple seconds. Extends the amount of time before having to re-hard cast one, which gives the chance for a new imp proc. Same for your soulburn soulfire, you don't have to soulfire immediately, you can hold it off since the soulburn is activated for like 15s.
You're doomguard damage was pretty low also, only 180k. You can get an easy 300k++ out of it by popping him at the start, at the optimal time. The optimal time to pop the doom guard for most fights is during the opener, when you have as many procs proccing (power torrent, lightweave, WoU stacks) while your pre-pot is still active.
My last comment is....why did you life tap? As destro, unless there's multi dotting, aoeing, or a mechanic that takes away mana, we don't need to life tap. Ever. So I seriously doubt you actually needed to life tap for a total of 63k+ mana. :x