Necros, especially on a support role provide a good range of group utility. The best thing about the Necro is, even if you already have other Necros in a group, using a Necro Tank still has a valuable function within that group which is not the case for most other classes. When we look at Trials, you should always aim for 3 Necromancers group. If this is not the case, then this is also great time to consider a Necro Tank.
The initial benefit of using a Necro Tank is for the Colossus Ultimate. This increases the damage taken by enemies and is a valuable tool to help your group kill enemies quicker. You can also utilise the Necro for gaining Ultimate at a much faster rate than other classes and this means you can Colossus enemies more often. This is achieved by using Necro Skills that create corpses and then absorbing those corpses, along with the corpses from killed enemies to gain Ultimate with Necrotic Potency.
The second main reason for using a Necro Tank is the group buff of Empower that is provided via Empowering Grasp. This skill does the same thing as the Galenwe gear set but is 1 skill rather than a 5 piece gear set. This is what makes Necro Off Tanks so vital for a lot of Trial groups nowadays, as they can keep casting this skill at their group to buff all Light Attacks by 40% which can yield a group DPS increase of anywhere from 2-6% depending on how good they are at Light Attack Weaving.
A few other great things from they Necro include their ability to provide a number of group synergies, and Boneyard which provides a synergy is also an AoE Major Breach skill which is great for situations where you deal with a number of enemies. The Necro can also create their own synergy with Avid Boneyard which can be extremely useful if you are using Alkosh.
The Necro has a good self heal with Hungry Scythe, this scales off Max Health meaning the more Health you have the bigger heals you get. Its a strong heal which is even better the more enemies you hit with it. The major downside of this skill is you need to hit something for it to work, so sometimes it can be a real problem if you are in a fight that requires lots of movement as you may fall into situations where you cannot heal.
Necros can also use Spirit Guardian to gain some healing over time, which is nice but it can sometimes go and heal allies in your group as it is designed to heal whoever has the lowest health. The great thing is that it provides 200 extra Magicka Recovery via Necro passives and it also provides additional damage mitigation.
Necros also have Agony Totem which can be a really nice multi-use skill where it fears enemies but also provides a synergy and applies AoE Minor Vulnerability.
A few things to consider when choosing to play a Necro is that healing can be difficult as mentioned already if you are in situations away from enemies. Sustain can be a particular challenge as they are no skills that will give back resources while blocking. Crowd Control is not the best although better than some classes, combining Silver Leash and Agony Totem doesn’t work. If you chain an enemy they cannot be feared afterwards so it doesn’t work as well as the DKs chain and talons and using Beckoning Armor is very ineffective as it take too long.
Overall I believe the Necromancer Tank is a very strong option to consider using, especially as an Off Tank. If you are in a Trial group with less that 3 Necros they are extremely useful, but even if you have 3 already, they use a completely different skill set to a DD which means they offer a whole new function in group content. It requires a bit more effort to play a Necro and the playstyle is different to other classes but the benefits are worth it.