Discussion Showing a sign when performing a skill.

Hello community!

In some servers, when a epic raidboss (Antharas, Zaken, Valakas, etc.) performs a certain skill, a sign or message appears on the center of the screen.
(The message says something related to the epic we are dealing with) 

My question is: 
Is it possible to edit from client side and trigger this kind of message/sign when I perform a skill or, when another mob (aside from the epic ones) do a skill? 
I found those messages on "NpcString_Classic-eu", But it doesn't "tell" me how it triggers or from where. 

I hope you get my idea.

Thank you!