ICC shadow priest wanna be

Hello ladies and gentlemen,

I've been playing on this server for a few years, I don't have a lot of experience in icc, time didn't allow me to spend 2-3 continuous hours in front of the computer, but in the future I hope to succeed, and I have some curiosities.
I have a priest, I like the class, both pvp and pve, the problem is that I don't know exactly what a shadow priest should do in ICC, or RS, I mean the essential things that I should do, not about rotations or dps.
For example, if there is only one priest in a group of icc10, what obligations does he have, so that the raid benefits 100% of the potential of the class, I am referring here to, for example, to clean the room in front of the PP, or shake undead before the Prince, or at ph3 LK to block vile spirits. What other things should he do? fearward, dispell, etc, and most importantly, where and when?

p.s. I looked on YouTube at sp raids, I read other shadow priest guides, but I didn't find anything like this. thank for yours advance.