See Ramchal’s Ma’amar Reisha v’Sofa (found in Adir BaMarom 2, pp. 35-60) and his Peirush Ma’amar Arimat Yadi (Ibid. pp. 61-92) for his discussion of everything returning to Elohut. See Ba’al HaSulem’s treatment of it in his Introduction to the Zohar, and The Leshem at various points.
I’m going to start blogging from Ramchal’s Sod HaYichud (found in Ginzei Ramchal pp. 264 – 271) in which he explains the various elements of his all-important take on Yichud Hashem.
I warn you — this is not your father’s Kabbalah.