Which term best fits the blank: Three brothers, princes of Ras, accepted the invitation and founded a from which many of the Russian princes of the present day claim descent. *
Dynasty
Caste
Covenant

Dynasty

Explanation:

Three brothers, princes of Ras, accepted the invitation and founded a  dynasty from which many of the Russian princes of the present day claim descent.

The meaning of the word dynasty is said to be a line of hereditary rulers of a country, province, district, etc.

This is the best term that fits the sentence because, from the sentence, three brothers accepted an invitation and have descendants till present times.

d. dr. francis collins