Complex sentence: Despite the urging of Elrond and Círdan, Gil-galad's lieutenants, Isildur did not throw the Ring into the fires of Mount Doom, but instead claimed it as an heirloom for his House and as weregild for the deaths of his father and brother.
Split into simpler sentences: Despite the urging of Elrond and Círdan, Gil-galad's lieutenants, Isildur did not throw the Ring into the fires of Mount Doom. Instead, he claimed it as an heirloom for his House, and as weregild for the deaths of his father and brother.