smoldering tendrils in stygian veil

how do you defeat these in relatively safe manner?

Their attack almost wipes my entire party every time. I brought seven heroes and barely managed to kill one then a hundred enemies popped up and finished me off.

Flag your heros at the last trench,pop the quest and let the heros deal with the spawn. Move round and kill the next smouldering then flag your whole party back out of range beyond the torment claws. Do that for them all then /dance at chest.

Pain inverter. Drop a PI on the tendril and it should blow up in a single slash if it hits 3 or more people.

You need protective bond on you if you do this, though, otherwise you will all die.

For the tendrils, I flagged heroes away from me then had one of them cast protective spirit on me. I went in (warrior) and took down the tendril. As soon as it was dead, ran back and flagged heroes away from the spawn. Once they stopped following, I slowly made my way back. Before fighting the next "big" tendril, clear out the "small" ones that spawned.

Cast Prot Spirit on yourself, run in (make sure you don't run in a straight line from where hero's are flagged, move over a bit, then attack from side) kill it then grab argo of spawns, pull them to hero's and then do it again, you shouldn't have to kill any claws, when moving to next one just keep flagging hero's and they won't take any dmg.

torment slash (the skill the tendrils use) attacks in a straight line toward the position you are at for the start of the skill. if you move left to right while the skill is casting or after, the projectile will miss you. moving back and forth tho, you will get hit, it has to be side to side

interrupt that skill and flag away your party just before it dies

This was one are of DoA that I didn't struggle with (I am a Ranger).

Flag party well back.
Stand on a hill and shoot tendrill till it dies
Run away
Lure mob to heros and kill them
Pick off the Torment Claws from safe distance

Repeat x6

Have someone bring and maintain Shelter.

Don't let the tendril hit the Shelter.