Complex sentence: Defending a land under siege, he successfully defended the Númenórean exiles, the earlier settlers, and the indigenous population of Gondor until Elendil, Isildur and Gil-galad summoned him to take part in the campaign into Mordor.
Split into simpler sentences: Defending a land under siege, he successfully defended the Númenórean exiles, the earlier settlers, and the indigenous population of Gondor for a time. When Elendil, Isildur and Gil-galad arrived, they summoned him to take part in the campaign into Mordor.