Where does “*tъ” come from?

*tъ (Proto-Slavic) comes from Proto-Balto-Slavic tas, from Proto-Indo-European só — this; that.

Ancestry of “*tъ”, step by step

Step	Language	Word	Meaning
1	Proto-Balto-Slavic	tas	that
2	Proto-Indo-European	só	this; that

*tъ — Etymology, Origin & Word History | Etymology Explorer