The Kalabros Manuscript is a book that contains incantations allowing one to use the Spear of Destiny.
Eobard Thawne tasked Damien Darhk and Malcolm Merlyn with finding the Kalabros Manuscript. However, due to the Legends' initial arrival in France 1917, he brought Darhk to France leaving Merlyn to track down the manuscript. After Mick Rory gave the Spear of Destiny to the Legion of Doom and they went back to the Vanishing Point lair, Merlyn returned with the manuscript and chanted its incantations allowing the Legion to rewrite reality into their ideal version.
After rewriting reality, Thawne later destroyed the Kalabros Manuscript meaning only he knew how to use the Spear.
When the Legends' of the alternate reality returned to 1916, Thawne followed them and pulled Merlyn away from finding the manuscript in order to fight the two groups of Legends meaning that the manuscript was technically never found by Merlyn.