Where does “*-ьňà” come from?

*-ьňà (Proto-Slavic) comes from Proto-Slavic -ja, from Proto-Indo-European -yeh₂.

Ancestry of “*-ьňà”, step by step

Step	Language	Word	Meaning
1	Proto-Slavic	-ja	Suffix creating feminine action nouns, usually...
2	Proto-Indo-European	-yeh₂	—