Aran Skill Tables

I don't really understand how Tempest works or exactly what it does.
The description says it attacks 4 times?
for 40 seconds?

In every video of it being used I still can't seem to figure out what's so good about it.
There was only one where I actually saw the damage, and it was something like 141k to an Oblivion Guardian.

Tempest 1 hits normal mobs when you touch them after the skill, and takes a chunk of HP off a boss but shows no damage. The 4 hits is only related to your combo. You have 15 mobs in front of you, you get 15*4 on your combo count.

It is so far a useless skill and not worth losing your combos for what it gives you.