auxiliary verb, godan verb (-ru, irregular)
it is that ...; can say with confidence that ...
Emphatic explanatory ending, stronger than なのだ. Used in formal or written Japanese to assert a conclusion or fact with confidence. Often appears in expository or argumentative contexts.
See also: なのだ
この問題は、慎重に検討する必要がある。なのである。
This issue requires careful consideration. That is indeed the case.
彼がリーダーに選ばれたのは、実
The reason he was chosen as leader is, assuredly, because he had the ability.