The Osireion lies directly behind the Temple of Seti I, partly below ground and very different in mood from the decorated temple halls. Massive stone blocks, water, shadow, and a stripped-down plan make it feel closer to a tomb than a temple.
Access can vary, and sometimes you will see it only from above or from a limited viewing point. Even then it is worth seeking out, because it gives Abydos its darker, more mysterious edge.