Terrestia provides an alternative 'backdrop' against which to run your own or published adventures. What it provides over and above a standard 'Elfs, Dwarfs, Orcs' world is a sense of discovery about the true scheme of things and a departure from the default Cosmic Laws of fantasy settings. Magic is scarce and can only be learned via Shadowmancy or given by a Daemonic patron. The worshipped Deity 'Nefaria' is merely a demon herself, and the true force for good in the universe is almost unaccessible except by the most extreme meditationists. True goodness, if it can be found, lies only within the hearts of rare men and has little authentic support from above.
Nonetheless your campaign need not focus obsessively on these and other 'higher truths'. This knowledge, if obtained by the players, should be suprising and shaking.
Rather the secret principles of Terrestia should provide for unusual scenes within day to day adventuring. 'The Dreaming' might be hinted at by seeing a person dissapear from sight, or returning to a village that isn't there. The true power of Shadowmancy might be preluded by visiting a forest which is utterly drained of life. A prisoner suffering visual and tactile deprivation might eventually have a vision or hallucination of the true Goddess. The Hand of Cadavalia may provide the adventurers with much freelance mercenary work before their real objectives are discovered.
Thus the concepts of Terrestia serve to provide your players with unusual encounters, startling sights and bizarre experiences. A sense of coming closer and closer to final truths will stimulate your players and provide hours of mystery and intrigue.