One day, Slaanesh looked at how all the other Chaos Gods had their own unique character. Wanting a piece of the the action, he/she/it created Shalaxi Helbane, a Keeper of Secrets made specifically to peg the champions of other Gods, with Bloodthirsters getting the absolute brunt of it. To this end, Slaanesh also gave Shalaxi a cloak with tassels that lengthen and morph into tentacles to ensnare its opponents.
It has done this numerous times, and the almighty goat has fought Skarbrand himself twice, both times
Shalaxi got cockblocked the forces of Order interrupted the duel, making Shalaxi more eager than ever to best their nemesis. Despite this, Shalaxi still doesn't have the plot armor necessary to best GW's pet faction, and was banished into the Warp by the Grey Knights during a confrontation, which greatly displeased Slaanesh and threw them into the Palace of Punishments , dayum.
Eventually, Slaanesh thought the torments were enough, and dispatch Shalaxi to stop and destroy the Ynnari by any means necessary. So far, they've assembled an army of daemons and mortals to help them in this, and is having a blast proverbially fucking with the Ynnari.
This all came to a head in the Battle of Iathglas, where a colossal battle between Shalaxi's and the Ynnari's armies. A gigantic battle ensured in the climax where the best of the Ynnari was gathered for a duel with Shalaxi, and it seemed like they were beating them, but turned out it was just a very powerful "glamour" (an illusion strong enough that it seemed like it was a solid entity) to test their strength and a bit of a flex on how powerful they truly were. This was all a trick and Shalaxi left after a laugh, proclaiming that the Ynnari were just small fry to them and not worth their time, putting into perspective on:
- How powerful Shalaxi actually is and that even at their best, the Ynnari probably wouldn't be able to withstand them personally taking to the field.
- How overpowered/inconsistent the Grey Knights are in fluff.
Of course, there's also the possibility that Shalaxi was just straight up lying.
Other than this, they are known for their fabulous hair reminiscent of David Bowie.