The Necrotic affix is a pain to deal with, it lasts for 9 seconds does damage over time and cuts shields and heals. Even worse it stacks and the counter is refreshed with every hit. Its one of those affixes that encourages your party to also pitch in and help the tank deal with managing mobs. (Your not the only tank that does not like it since you need the party to help you do your job here and its more running than tanking)
Before you even start aggroing mobs, you need to plan where they will path through and where you can kite to safely. Always kite when you get off enough parries that you can easily drop your stacks. Now the number of stacks you can manage changes depending on the tanks and healers, so once you notice that your healers are having difficulties keeping you topped up and safe try kiting (or simply running) away while asking your party to throw down some aoe cc (enough for you to spend upto 9 seconds not getting hit) so that you can drop the debuff get healed up and start tanking again. Using death and decay is good as the slow helps out, ideally you spam it as often as its up and try kiting away from the mobs so you gain stacks more slowly but there is not much more that you can do. In really clutch situations pop a lightfoot potion along with your aura and just run.
[Note that for mobs that are immune to stuns and snares all you can do is run and hope that either they get burst down or you manage to drop your stacks before it gets overwhelming. Also note that pulling smaller groups of mobs in sections where you can reset your stacks in-between is also a good idea if your not already doing it]
*There are ample guides on youtube which might be able to help you better than a text explanation