What the topicstarter meant, was that some Summoning songs have a sad (depressive, miserable) feeling in them. I agree with this statement.
Best, inspired, thoughtful and valuable albums of Summoning are: Lugburz, Minas Morgul, Dol Guldur, and Nightshade Forests.
Some songs have an overall sad mood: Morthond, Khazad Dum, Wyrmvater Glaurung
Some songs have a 'nostalgic' feel in them (even if the term sounds absurdist): Flight of the Nazgul, The Passing of the Grey Company, The Legend of The Master-Ring, Kortirion Among the Trees
And most songs have an epic grandeur journey feeling: Through The Valley Of The Frozen Kingdom, Lugburz, Nightshade Forests, and the rest.
Strong sense of melody, intuitive compositional skill, work on structure, and first of all having concept (ancient, fantastic) in mind - are factors for which Summoning will remain forever one of metal's top achievements.