Complex sentence: Rishanger quite likely wrote the "Opus Chronicorum", a continuation from 1259 of Matthew Paris's "Chronicle", and practically a history of his own times from 1259 to 1307, which is both a spirited and trustworthy account, albeit in parts not original.
Split into simpler sentences: Rishanger quite likely wrote the "Opus Chronicorum", a continuation from 1259 of Matthew Paris's "Chronicle". In effect it is a history of his own times from 1259 to 1307, a spirited and trustworthy account, albeit in parts not original.