ending vs happening

ending and happening both are nouns.

Word	Noun	Adjective	Verb	Adverb
ending	Yes	No	No	No
happening	Yes	No	No	No

As nouns, happening is a hypernym of ending; that is, happening is a word with a broader meaning than ending:

Other hypernyms of ending include natural event, occurrence, occurrent.

ending (noun)	happening (noun)
the act of ending something	an event that happens
the end of a word (a suffix or inflectional ending or final morpheme)
the last section of a communication
event whose occurrence ends something
the point in time at which something ends