Where does “-ь” come from?

-ь (Ukrainian) comes from Proto-Slavic -ь, from Proto-Indo-European -is — Forms nouns from verbs.

Ancestry of “-ь”, step by step

Step	Language	Word	Meaning
1	Proto-Slavic	-ь	Forms nouns; From adjectives; From participles
2	Proto-Indo-European	-is	Forms nouns from verbs