How to Find Thiollier and Complete his Quest

First Meeting: Find Thiollier at the Pillar Path Cross site of Grace, southeast of Castle Ensis. From the Graveyard Plain site of Grace, head east to a path, then follow it north/northwest to the Three-Path Cross site of Grace. Continue northeast over the Ellac Greatbridge toward Castle Ensis. Just before the castle, take a path southeast, then a drop-off further southeast. Continue southeast to find the Pillar Path Cross site of Grace and Thiollier sitting on a rock. Speak to him to unlock him as a merchant selling poisons. Exhaust his dialogue.

Second Meeting: Travel to the Main Gate Cross site of Grace, west of Belurat, Tower of Settlement. From the Three-Path Cross site of Grace, take the western path north. Speak to the NPC (non-player character — anyone the game controls, not you) Moore and exhaust his dialogue until you can ask about Black Syrup. This option only appears after speaking to Thiollier. Return to Thiollier at Pillar Path Cross, hand over the Black Syrup, and select the dialogue option “I’m weary of life” to receive Thiollier’s Concoction. Do not drink it. After this, more can be purchased for 30,000 Runes.

Third Meeting: After breaking the Great Rune, head back to Thiollier at Pillar Path Cross. He will express a desire to see St. Trina one more time.

Fourth Meeting: Travel to the Stone Coffin Fissure. From the Cerulean Coast site of Grace, head south/southwest to the southern tip of the Cerulean Coast. In the southeast of the peninsula is The Fissure site of Grace and a tunnel to the Stone Coffin Fissure. This area is blocked if the Great Rune has not been broken. Progress through the area until you reach the Fissure Depths site of Grace.