At least one of the Vortex modifications are highly recommended since the initial range of Clustering Maelstrom, is by comparison much lower.
Regardless of mods or other enhancements, Clustering Maelstrom works best when there are lots of enemies caught in it.
However, take note that if you prefer to use Clustering Maelstrom on small groups of enemies, the Gravitation modification may better suit your needs than Chaos.
The " +% damage of enraged players" bonus provided by Purifying Chastisement (and equipment) increases the damage of Maelstrom.
Thus, an Inquisitor who likes to get surrounded and take damage can use Clustering Maelstrom to suddenly wipe out a crowd.
Inquisitors can "herd" enemies by running around, getting enemies to see him and gang up in a mob and then suddenly unleash Clustering Maelstrom for a huge payload.
Against bosses who are accompanied by lesser monsters, the Inquisitor can use Clustering Maelstrom to cause a (sometimes fatal) burst of damage.
Clustering Maelstrom can quickly charge up Soul Reaver. This is due to Maelstrom's potential to kill huge crowds.
Paralyzing Dread can be used to slow down nearby monsters who may try to escape Clustering Maelstrom.
The Bleeding damage from the Gash modification is applied to all monsters in range of Clustering Maelstrom as soon as the spell is cast, rather than after the monsters are drawn together. This mod is a good way to apply Bleeding damage to single targets as well as large groups.
Levin Array works well with Maelstrom, since it can hit crowds as well.
Console Version Note: This combo leads to very frequent game crashes/freezes.
Ruthless Mutilation can be used before or after Clustering Maelstrom; this is useful for either hurting & slowing down monsters before Maelstrom or killing the survivors.
Raving Thrust, Clustering Maelstrom's little brother, can be used to push an enemy into a crowd prior to unleashing Clustering Maelstrom for extra impact.
Eruptive Desecration can be used after Clustering Maelstrom to take advantage of any possible dead bodies. This can help finish off any survivors of Clustering Maelstrom.
Mortifying Pillory can be used to cause enemies to attack an outlaw. The Inquisitor can then follow up with Clustering Maelstrom.
Advanced Combo: For a taking out crowds of high HP enemies -
Begin with Levin Array to not only damage a large potion of enemies, but to also apply a continuous damage effect to them as well.
Next, use Clustering Maelstrom to smack the enemy crowd into each other.
As soon as the enemies start to slam into each other, use Ruthless Mutilation, to slice through all of them at once!
Pros and Cons
Can be used to quickly annihilate huge crowds of enemies.
Easy to incorporate into a number of Inquisitor combos.
It can send hundreds of dead bodies flying in all directions. :)
Useless against single targets (requires for there to be at least 2 enemies for there to be a collision).
High regeneration time.
Range of Clustering Maelstrom is low without at least one of the Vortex modifications.
Fast enemies can run out of the Maelstrom's range, before they are pulled into collision.
Lack of instances where it can be used effectively on bosses.
Console Version: Using Clustering Maelstrom on large groups can lead to frequent crashes.