I am on the PTS testing out this skill. I looked at what the skill looked like before the 1.6 changed and what it looked like now. I also tested the skill and its morphs and see how they work. I also compared it with the other skills and passives in the skill line Siphoning Skills.

Now in general this skill is not useless. I was playing around with the Morph of this skill, Malefic Wreath, and the skill Crippling Grasp. These skills work well with each other. One stuns and does damage after it is broken and the other does burst damage, immobilize and reduce the movement speed of target while also boosting your speed. You can use Malefic Wreath at the beginning of the battle and use crippling grasp to delay the target long enough to get off another Malefic Wreath or another attack.

Now I believe the problem of this skill is that it does not know what it is and most people do not know what this skill does. I also think this skill tries to do too many things that it does not do any of them well. Here is a comparison of what you change in 1.6.

Before

Cast time:1.5 seconds
Stuns enemy for x seconds, effect is broken by damage from any source. After effect ends enemy takes x Magic Damage over 5 seconds.

After

Cast time: 1.2 seconds
Shackle an enemy in torment stunning them for 30 seconds and dealing [x] Magic Damage over 5 seconds when the effect ends.
Stun is broken if the enemy takes damage.

So the only changes you did to this skill is that you boost how long this skill stun last and lowered the cast time with a little bit of flavor text added. Now the only pro I see for these changes is the lower cast time but added stun time does not really help this skill at all and before I get into why it does not lets look at the morphs.

Prolonged Suffering

Shackle an enemy in torment stunning them for 30 seconds and dealing [x] Magic Damage over 7 seconds when the effect ends.
Stun is broken if the enemy takes damage.
Damage over time last for longer.

Malefic Wreath

Shackle an enemy in torment stunning them for 30 seconds and damaging all nearby enemies when the effect ends for [x] Magic Damage plus 2% for each second the target was stunned.
Stun is broken if the enemy takes damage.
Deals area damage when the effect ends.

Prolonged Suffering is just like the base skill Agony but with a longer dot. The defining trait for both of these skills is that you have a stun and a dot. Stuns work on on all mobs that are not immune them. So this skill is suppose to work on mobs but since it is a single target skill it only work on one mob. Now the problem arises why give a skill which only stuns one enemy more time for the stun, when most mobs in the game becomes a problem when there is three or more of them. This skill is too situational to be use in this way.

Thus, no one would not want to use this on their bar. Plus this skill breaks on damage when most people deal with multiple mobs they use aoe skills so they would not have any use for this skill on their bars.In pvp, this extra time is not going to help because they will easily cc brake this skill. So the extra time really does not help the base skill and this morph. Now the other benefit of this skill is the dot. The dot is ok and does decent damage but that is it. Now when you put a stun that takes a long cast time, takes damage to break, can be easily cc break, and with mediocre dot. For the 1.6 change you should of focused on cast time and the dot damage instead of increasing the stun time or gave it so more utility.

Now the next morph is completely different then the other morph and base skill. Malefic Wreath has the same stun time as the other morph and base skill but when it is broken it does aoe damage and it does 2% more damage for each second the target stays stun. At a glimpse of this morph it seems that it benefits from the longer stun time.Problem with this is that you have to wait for the for 30 secs for full damage. The bad thing about this is the extra damage is less than the initial damage and does not do enough damage.

I thought the point of the 1.6 change was to make combat quicker. No body is going to wait for the skill to do more damage when they can easily do more damage with other skills. Another thing that is bad about this morph is it does aoe damage when the skill is broken. The problem is if you use this on mobs and your far away the mobs that are not stun will agro to you and when the skill breaks the damage only effects the target and none of the other mobs. The only benefit of this skill is to use it when you are close to the enemies in battle while getting hit while channeling the attack then you have to break yourself in order for it to do damage to all the enemies .

The only effective way to use this skill and its morphs is to stun a enemy and than break the stun your self, since the timer is too long, to gain the benefits of the dots and aoe damage from the wreath. The skill wording makes since when the skill actually does damage while they are rooted but instead if only does damage after it is broken. Which is not bad but the extra stun time really does not boost the utility of the this skill since now some mobs can cc break this skill and you cannot use the stun effect on enemies immune to stun, which are the actually enemies you want to use the stun on. Malefic Wreath would be better to hold multiple enemies than just one if it does aoe damage. If not it should just use single target damage since it most useful that way.

Last is the skill line synergy. The skill line is dedicated to resource gathering from enemies while sustaining our self better. Siphoning line has skills like soul shred,strife, and leeching strikes that siphons health. Cripple which siphons movement speed. Drain power that suppose to siphon vigor from the blood dealing magic damage while we gain buffs in attack. The passives is all about increasing magicka, gaining ultimate from potions and skills and increasing healing done. All skills follow the theme of gaining resources from enemies and other sources except Agony.

This skill feels out of place of this skill line. It does not siphon anything, its does not allow gaining or any resource and this skill does not synergize well with other skills other than having it on your bar allows you to gain 3% healing done thanks to a passive but the skill does help with healing others or self healing. So it pretty much useless on a healing spec. I really want this skill to better but it seems ZOS does not know what they want this skill to be. It seems you miss a great chance to make this skill better.