how to change the skill "possess"

[Synchysi]

All this being said, can I change Possess in all together to something similer to Cecil's Dark Wave?

Quite easily. Just create a spell that mimics Dark Wave (you'd probably have to create a new special effect for the self-damaging aspect) and point Possess at that spell ID.

Shock does the same thing, pointing to a spell called MegaHit I believe.