I already have the egg I need, but I'll be happy to help you with the EnrAegis. It's rather simple - pick up a random egg and kill it. Then go to your selected Aegis and select the "Enrage" action. It will change its appearance.

If you want to keep it that way, you don't need to do anything else. No need for constant killings, as some people assume. If you want to change it back, you just wait until there are no dead eggs or gravestones on your scroll, and change it back with "Pacify".