Complex sentence: He elaborates mania grandiosa type plans to exterminate Russia forever and build on the ruins of great empire Gross-Estland (Great Estonia), which would incorporate all former Fenno-Ugric territories, including Saint Petersburg.
Split into simpler sentences: He elaborates mania grandiosa type plans in order to exterminate Russia forever and build upon the ruins of the great empire Gross-Estland (Great Estonia). The empire would incorporate all former Fenno-Ugric territories, including Saint Petersburg.