I have an idea but need help

My idea involves the skeletons and the headless horseman when you die by the horseman you character gets turned into a skeleton and fights with him. Is there any way to build this into a map or any idea how to do this without getting a server and putting it as a plugin? I want it to be built into the map and ill be happy to pack it up with Packrat.

Do you think this idea can be possible to do for a Tf2 map to have is so please give me a guide.

(ps if anyone can make an alien boss model and help me to get it to work ill be happy to credit you with the creation aka I want something like this
View: https://youtu.be/Sw3ZdIEpwgE?t=79
this type of battle to be mean and ugly)

If you need me to elaborate please tell me what I need to elaborate on.

 

Definitely need a server plugin for this.