Complex sentence: When Béla were informed that the Polish and Rus' armies entered to Hungary, he assembled a meeting of the barons where all the participants were killed who did not want to declare Boris a bastard result of infidelity.
Split into simpler sentences: When Béla were informed that the Polish and Rus' armies entered to Hungary, he assembled a meeting of the barons. All the participants were killed who did not want to declare Boris a bastard result of infidelity.