You will only receive this quest if you have not completed the "For the Advancement of Learning" quest or if you have not succeeded in dissuading Keira from meeting Radovid. Then, when you talk to Triss during the "Final Preparations" quest she will inform you that Keira is dead and that she wants to retrieve her body.
Life, like art, abounds in tragedy, and sadly the ending of Keira Metz's story added to its cornucopia of woe. With the notes on the plague she took from Mouse Tower in hand, she presented herself to King Radovid. She was counting on obtaining a royal amnesty in return for the possibility the plague could be turned into a weapon to wield against Radovid's enemies. But she forgot one important detail - never deal with a devil, and especially not a devil backed by armies and with a fierce hatred of sorceresses.
Radovid laughed in her face, arrested Keira and had her impaled on a pike. She died in hideous agony, screaming curses against Radovid and his kingdom that echoed through the streets of Novigrad.
When Geralt returned to town, Triss asked him to help retrieve her body and give it a proper burial.