Question on big pulls?

[TouchandFeel]

For starters, don't use your defensive abilities back to back like that because you will end up with the exact situation you are asking about where you blow through them all too quickly and leave yourself high and dry.

As a tank you need to know and learn the flow of your cooldowns and how they relate to the speed and cadence of a dungeon or whatever content you are doing.
Since you seem to be asking specifically about dungeons and big pulls on DRK we'll focus on that.

First off, TBN is your primary defensive ability as DRK and you pretty much want to be using it constantly during big pulls. As soon as it's off cool-down back up it should go. DO NOT be treating it the same as your other cool-downs where you just cycle through it once one of your other defensives runs out. It has a super short recast for a reason and you use it IN TANDEM with your other defensive abilities. The only times that you may not want to use TBN when it is available is: when you are down to the last mob or two of a pack and they will be dead any moment because the damage at that point is negligible and you don't want to waste the MP without the shield breaking, and when you are still initiating the pull and running through/rounding up the mobs since all your defensives are most effective when you have more mobs hitting you so it's better to wait until you've stopped the pull and have the mobs converging on you before throwing up TBN and/or any other defensives.

For the most part TBN will be the first and likely last defensive used in every major pull. As soon as you stop the pull to settle where you and your party are going to actually fight the pack, throw up TBN and then quickly pair it with another defensive like Shadow Wall, Rampart, etc. TBN and the other defensive abilities that are % damage reduction pair incredibly well together because the % damage reduction affects how much damage the shield of TBN is taking, making it even more effective and resulting in overall more mitigation achieved per ability usage.

Which ability to pair with TBN will depend a lot on the pull itself. In pretty much all dungeons the wall to wall pulls come in approximately 3 sizes; small, medium and large. For example in many dungeons the first pull tends to be on the smaller size, with maybe only 2-3 smaller groups of mobs. In instances like that you can usually just lean on TBN and not have to layer in another defensive, but if the group is slow on killing stuff or you are undergeared and taking a lot of damage from the small packs you may want to layer in one of the weaker defensives with a shorter cooldown like Rampart or Reprisal (remember this one is AoE now so can be used in large dungeon pulls).
The medium pulls can usually be handled with TBN plus Rampart, then followed by TBN+Reprisal+Arms Length, or similar weaker pairings and the large pulls can often require TBN+Shadow Wall upfront and then cycling through TBN + whatever weaker defensives you have available.

Also keep in mind you have Abyssal Drain which while an offensive ability can also heal for a lot of HP when used against a large pack and should be thought of and have it's usage planned out like a defensive ability and with it's 60s recast you should be able to use it every or every other big pull at least once. Just make sure to time it's usage well like in a gap between having defensive abilities up when the incoming damage will spike and definitely not when you are sitting close to or at full HP. That should be obvious.

Speaking of offensive abilities, you need to also plan out your offensive burst ability usages to coincide with the pull sizes and your defensive ability usages. Dead enemies do no damage, so the faster that you can kill off the mob packs the less damage you will take. Matching up your big bursts like Delirium with the really big packs will help you kill off more of the pack before your initial TBN+Shadowall (or whatever) is gone, which reduces the damage the rest of the pack will do and reduces the need of having to utilize additional defensive abilities asides from TBN. Blowing your big burst abilities on small or weak packs only to shave a few seconds off how quickly the DPS players would have killed the pack is a waste. Use them where you can maximize their effectiveness and where it will help kill the most mobs more quickly and therefore prevent the most amount of damage from them.

While I am saying to make sure to not waste ability usages and to use them at the right times, that does not mean that you just sit on them, saving them for emergencies. Sitting on your abilities and not trying to maximize their overall up-time in a dungeon is just as much a waste, if not more than using them at times when they are less effective. It's all a balancing act that just requires the tank to learn their abilities intimately and to get the feel and timing of the content, creating a mental map of the encounters and what abilities to match with them. It may take a of practice to get used to doing this but after some time it just becomes second nature.

Lastly the most overlooked "defensive" ability in a tank's repertoire, Sprint. A lot of players will tell tank players to use this or that defensive ability while you are doing the pull in order to reduce the damage you take during that time. Don't do that. You are just wasting a defensive that you could be using after you finish the pull and are actively engaging the pack of mobs.
What you should do is just learn to do fast and effective pulls with Sprint. If you use Sprint properly you will pretty much negate close to all damage during the pull by just simply moving faster than the mobs which will keep enough distance between you and them so that they can't hit you and do any damage. Basically you need to hit Sprint after tagging the first group of mobs and then just go, running straight to the next group and moving straight by or through them so that you can tag them all with a single Unleash as you pass them and keep on running to the next group to rinse and repeat until you reach the end of the pull. As long as you are sprinting and don't stop, turn around and the like during the pull, the delay of the mobs registering agro on you and then going to engage mixed with the speed of you sprinting should result in them always being a decent bit behind you and out of reach for them to hit you. If an overzealous DPS manages to rip hate from a mob or two during the pull, don't sweat it and don't stop and turn around to try to regain agro on those mobs. Just keep going. Any DPS that has a clue what they will doing will be right there running along with you and therefore shouldn't be taking much if any damage and should just bring the mob in for you to grab when you stop to fight the pack.

Now if the problem is that the party as a whole has atrocious DPS and things are just taking way too long to die which forces you to have to burn way too many defensive abilities per pack; well there is nothing you can really do asides from sigh, roll up your sleeves, resign yourself to an overly long run and do your best to make it go as fast and painlessly as you can.

[aodhan_ofinnegain]

Basically you need to hit Sprint after tagging the first group of mobs and then just go, running straight to the next group and moving straight by or through them so that you can tag them all with a single Unleash as you pass them and keep on running to the next group to rinse and repeat until you reach the end of the pull.

I would recommend where possible, to pop Sprint before you pull the first mob, as you gain 20 seconds of sprint, compared to 10 seconds if you're in combat. In some of the 80 dungeons I find the gap from wall to wall can be a bit longer compared to previous dungeons, and the extra 10 seconds can help speed things up, but of course it is a super minor detail, nice when you can get the full duration but whatever if you don't get it.

[TouchandFeel]

I would recommend where possible, to pop Sprint before you pull the first mob, as you gain 20 seconds of sprint, compared to 10 seconds if you're in combat. In some of the 80 dungeons I find the gap from wall to wall can be a bit longer compared to previous dungeons, and the extra 10 seconds can help speed things up, but of course it is a super minor detail, nice when you can get the full duration but whatever if you don't get it.

Good catch and a goof on my part. I actually hit sprint right before I get to the first group and then use Unleash, Total Eclipse or whatever as I pass through them. It all just happens so fast and I have relegated it to muscle memory so I think of it as one contiguous event in my mind, resulting in my jumbling of the order in the post.