Complex sentence: To do this, she must speak to the oldest Culture citizen still alive, one Ngaroe QiRia who had apparently been present at the negotiations setting up the Culture, who she has met 20 years before when on a placement with the Culture.
Split into simpler sentences: To do this, she must speak to the oldest Culture citizen still alive, one Ngaroe QiRia. Cossont met Ngaroe twenty years before when on a placement with the Culture, and discovered that Ngaroe had apparently been present at the negotiations setting up the Culture.