Drakenlord is a miniboss with an interesting mechanic. He can frequently be found in the Chaos Expeditions, Onslaught, Mastery and Incursions. He moves very slowly but will swing his sword twice every time he attacks and can call forth a shield of ice to protect him and chill, then freeze, then damage anything inside.
Drakenlord is a scary boss at first, he can raise his sword and plunge it into the ground summoning an ice shield, once his ice shield goes up he has very high resistance to damage (it appears to be 99% resists) and it can slow heroes then freeze then in spot and, after a short time, call forth a spectral copy of himself to deal a finishing blow. This final attack seems to always be a one shot no matter how much health you have. On top of that it can do the same to defenses but also enemies which can be helpful.
If he does get his ice shield up before you kill him you can bring it down by setting him on fire. This can be done with the Oil plus Fire damage combo creating an ignite. Alternatively, you can use the Burning Strikes shard to do the same. A word of caution, if he is ignited before his shield goes up then you will have to wait until the ignite wears off before igniting him again to bring down the shield. He is a threat while this happens, so stuns can hold him still while you wait.