In Robert Harris’ “Lustrum” the body of a sacrificed slave is found in Rome. In the novel the sacrifice was said to have been made to hold the conspirators to a vow of secrecy. Do our ancient sources have anything to say about Cataline’s use of human sacrifice or is this an embellishment on the author’s part?
It's unlikely, but it is mentioned in some of the ancient sources. Sallust's 'War Against Catiline' goes into most detail talking about rumours that apparently circulated accusing the conspirators sealing oaths by the drinking of human blood. Sallust is very doubtful about these rumours, even though he mentions them anyway, and I think he's right to be suspicious. If these rumours had even the weakest basis in fact, Cicero would've seized on them in his speeches against Catiline. We have 4 surviving, and in them Cicero accuses Catiline and the conspirators of every awful act under the sun, but never mentions human sacrifice.
If you want to read the ancient sources yourself, see here:
Sallust: http://penelope.uchicago.edu/Thayer/E/Roman/Texts/Sallust/Bellum_Catilinae*.html Plutarch: http://penelope.uchicago.edu/Thayer/E/Roman/Texts/Plutarch/Lives/Cicero*.html
Cicero's Catilinarian speeches: https://lexundria.com/cic_cat/0/