RE : Sith & dark side practitioners etc (see details)

What happened to the dark side users or Sith when the died? Did the return back to the force or? (Canon only)

They became one with Bane and died again

They merged with Palpatine. Somehow.

^ thanks Galan , also sorry not sure my self : when Sids died where do you think his Life energy went ? Did he merge into some other essence ie dark side energy via the son etc or?