Complex sentence: It is both the longest of the stories that comprise his Dream Cycle and the longest to feature protagonist Randolph Carter, and can thus be considered a culminating achievement of that period of Lovecraft's writing.
Split into simpler sentences: It is both the longest of the stories that comprise his Dream Cycle and the longest Lovecraft work to feature protagonist Randolph Carter. It can thus be considered the significant achievement of that period of Lovecraft's writing.