The consensus (1, 2) regarding this set seems to be that it's been F tier for several years ever since it was nerfed into oblivion (pun intended).
As an avid DoT enjoyer, I love the concept of this set because of its unique damage type, however it's simply not usable in its current state.
I know it's a sensitive topic because it used to be extremely overpowered, but I'd like to discuss a potential rework that could make it viable without breaking the game.
Here's how:

Current:
Damaging an enemy has a 10% chance to fire a Shadow Pearl at them, dealing 3% of the target's max Health as Oblivion Damage every 1 second for 6 seconds. The Shadow Pearl cannot be reflected. Effect can occur every 6 seconds. This can deal a maximum of 3000 Oblivion Damage per tick.

Issues:
The 10% chance to fire a pearl on damage makes uptime inconsistent and the global cooldown and duration limits the proc to a single enemy at a time, chosen at random due to RNG.

Suggested:
Dealing direct damage to an enemy fires a Shadow Pearl at them, dealing 4% of their current Health as Oblivion Damage every 1 second for 6 seconds. This effect can occur every 6 seconds per target. This can deal a maximum of 3000 Oblivion Damage per tick.

Reasoning:
Tying the proc condition to direct damage makes the DoT consistent but instead requires active player input to maintain instead of randomly proccing off of any damage instance.
Changing the cooldown mechanic from global to per-target will make it viable against multiple enemies at a time. Single target DoTs should be spammable.
The change from 3% max Health to 4% current Health will make it tick slightly higher at first, but quickly dwindle as the target takes damage. This results in an overall damage nerf of ~27% and is meant to balance out the proc and cooldown buffs.

Below is a graph showing the difference between the original and new damage models.
The "Tick" column shows the amount of damage instances taken by Sload's Semblance.
The "Original" and "Suggested" columns shows the current remaining Health of a hypothetical enemy after a certain number of ticks.
The "Effectiveness" column shows the suggested damage increase or decrease compared to the original proc at the current Health amount.

In PvE, the new and original procs are equal until the monster reaches ~70k Health.
In PvP, the new proc is stronger than the original until the player reaches ~30k Health.