This has led to the mistake of understanding being (as such) as a kind of ultimate entity, for example as idea, energeia, substantia, or will to power.
[1] Fundamental ontology is the result of Heidegger's decision to re-interpret phenomenology, as developed earlier by his mentor Edmund Husserl.
For instance, the thesis that a phenomenon is the essence of a thing could not be articulated solely with traditional concepts and terms.
Basically, all ontology, no matter how rich and firmly compacted a system of categories it has at its disposal, remains blind and perverted from its ownmost aim, if it has not first adequately clarified the meaning of Being, and conceived this clarification as its fundamental task.
By shifting the priority from consciousness (psychology) to existence (ontology), Heidegger opened a new direction for phenomenological inquiry.