This answer is just how I interpret it, the rest is - of course - up to you. The figurative meanings I can't quite guess, but I'll give you what I can make out.
Word 1: Promontory
Word 2: Diminishes
Promontory, Meaning: Promontory means a high piece of land such as a cliff or overhang on a mountain.
Diminishes, Meaning: Diminishes means to slowly fade away and vanish. His figurative meaning using diminishes here: '...Diminishes me, because I am involved in Mankind;...'. 'Diminishes me...' Means he's fading away metaphorically, not physically.