Chi Spell... or whatever...

Brucutu - Dreamweaver

A friend of mine is lvl 92 and his char has a special spell that can be activated once his Chi bar is full. It displays a little icon next to the yellow Chi bar.

My Chi bar gets full too, but I have no spells to use it with.

What is that, and how can I get this spell??

Bowlinbob - Lost City

spark eruption. You can get it after you finish your culti quests for your level.

Speak to a Barb Trainer to recieve it. Its not expensive at all. (maybe 30 spirit/30 coins?)

Edit: you can get it after your lvl 29 culti.

Requisite Cultivation Aware of Harmony
Discharge one Spark to add 150% weapon damage for 10 seconds.
Grants invinciblity for 1 second after the discharge.

Garmr - Harshlands

There are actually a few Spark abilities you'll be able to get. You can start getting Sparks at level 29 after you finish the Cultivation for Aware of Harmony.

When your Chi bar fills up the first time, a red diamond catches on fire next to it. That's your spark, and certain skills require 1 spark to use. One is Spark Eruption, which gives you 150% damge for a little while. There are other attack skills that you can get, there's actually one around level 29 but I forget what it's called. It's a linear AOE that I haven't tested yet. Its range is feeble (8 meters straight ahead) so I haven't found the right conditions to release it yet.

Note that when you do get Aware of Harmony, the spark will charge when your Chi bar fills. But if you use Flesh Ream or Devour, then the Chi bar drops and the spark sputters out. Fill the Chi bar up twice to get the spark to stay lit - once you empty it the first time you'll still have another full bar left. It's also nice for grinding since you don't run out of chi as quickly that way I think it's level 39 or 49 that you get a second spark added from a Culti quest, and then more powerful skills.

It's probably one of the later skills you saw your friend using.