I remember a series of books I read in the 1970’s about a feudal planet with two species of human. One similar to us, the other looking the same but possessing tentacles and the ability of taking energy or life force with a potential lethal force. Can someone remind me of the title or author of these stories?
I think you may be thinking of the Sime~Gen Universe, by Jacqueline Lichtenberg (sometimes together with Jean Lorrah.) To quote Wikipedia:
The Sime~Gen Universe, created by Jacqueline Lichtenberg, involves a future when humanity is divided into two subspecies, or larities: Gens, who look like ordinary humans and produce excess quantities of a life energy called selyn, and Simes, who have tentacled arms and must take selyn from Gens in order to survive. If the Gen is frightened and resisting when selyn is taken, the Gen dies. The series chronicles the history of the two subspecies' search for coexistence.
The split of humanity into Sime and Gen caused wars and the collapse of civilization, so the series is low-tech at first. From 1:
And finally a stability is achieved in which progress begins to be made. Basic industry, imports from far countries, some rudimentary archeology, and you have a civilization with the technological base that the Americas enjoyed around the year of the Ancient calendar, 1800. You also have the same uneven distribution of technology. There are mud-hovels where the only candles they have they make from the tallow of their own domestic animals. And there are houses with cast iron stoves - a fabulous and legendary richness at such a time. But it is a subsistence economy with no leeway for drought, plague, or war.