Development Assistance Many things, particles, damage entities ect...

I am currently working on a plugin called Exoskeletons. I need help with:

Particles. i need to make particles appear on the players back wherever he faces.

Entity damage. I need to make a method where all entities near me within 5 blocks will get damaged. preferably something that will damage all entities, and i need this to only trigger on a event, then after the event this will no longer be triggered.