Here's a Sathanas with moving arms, just like the FreeSpace 2 ending cutscene. You can control the arms with the trigger-subobject-animation, using the scripted type. I added a new turret, turret54, that is placed where the 4 arms will meet in the middle. The arms are also set to trigger when "turret54" fires, but the arms reset far too soon to make it look good. Maybe a kind hearted coder can make that work properly.
Also included are some additional things to help you recreate that subspace effect the Sath makes from the ending cutscene. There is a scripted sexp that automatically change the 4 arm turrets to a "subspace effect" beam, that fires the beams for a point right in front, and a scripted sexp that causes a subspace effect (made by mjn) to appear for a set duration. These sexps only work with the Sathanas, there are hard coded values in the script.
There's also a sample mission to see how it works. Pressing 1 will make the arms animate, pressing 2 will cause the beams to fire. I'd suggest that maybe just one or the other be used, since the beams look kinda wonky when they are up close, but maybe you don't mind it that way.
How everything is sequenced is up to you. In my tests and sample mission, a delay of about 6-7 seconds after the arms are triggered or the beam firing is triggered seems like the right time to start the real effect.
Here's some old videos of this in action. This was an earlier rev, but the basics are the same. The arm animation is more smoothed out better now. https://drive.google.com/open?id=154owMgeeqgTKhwTX1dRkOgcfZ5aeSGqG