WILLITH
Class Mastery
Lead from the Front - provides group wide major berkserk and major protection for 1 sec per 15 ult spent. Paired with Standard of Might which needs to be placed in group for everyone to benefit from so swapping this out for Magma Shell or War Horn in bosses where this isn't possible might be better.
Resolute Defense - increases the amount of damage you block the longer you block. Also has a chance to restore stamina
Booming Voice can also be used instead of Resolute Defense if you find you are lacking sustain instead.
Skills
There are a few complexities to the DK tank regarding skills. A lot of their skills are close range and single target which isn't ideal for the ice dragon or add pulls.
For add pulls, Elemental Susceptibility is swapped out for Razor Caltrops and Leashing Soul put on the front bar.
Heart of Flame gives you a ton of sustain by giving you a portion of your missing resources back plus a bit of health. A very solid bit of sustain for DKs.
Magma Fist can be used which provides a unique ~4% dmg taken debuff at the cost of being expensive in stamina and has a short cooldown. Can be used by a DK DD instead.
Hearth and Home gives you nice major protection and minor heroism and healing while in its area but can be replaced by Blood of the Green Dragon if you feel that you need a burst heal instead.
Igneous Weapons is a support skill for classes where major brutality/sorcery isn't easily gained, also lasts a minute so should only need to cast a few times a boss. Can be provided by a DK DD instead.
Dragonknights don't have a class source of minor resolve so I've put in Ulfsilds Contingency with dmg shield and gladiators tenacity to deal with that. Also will give you the mages guild passives which are good to have.
Sets
Since you'll be wearing both Lucent Echoes and Pearlescent Ward, one of these sets will need to be double barred. I've put Lucent on the body and Pearls on the weapons and jewellery but this can be swapped around.
Baron Zaudrus is used to get a bit more ult gen to support using Standard of Might. This procs from applying status effects to enemies so Incinerate is used to help apply burning in add pulls and Elemental Susceptibility during boss fights should be all you need for proc conditions.