The Great Beggar was a devout worshiper of the Prophet Lebioda around 1203 in Toussaint. He lived inside Lebioda's temple and constantly preached about how Lebioda was the true deity and all others were wrong, upsetting the local villagers to the point they'd attack the temple on occasion. Despite this, he continued to preach about Lebioda.
On 14 Feainn 1203, he was visited by Merten, a witcher from the School of the Manticore who was on a pilgrimage to renounce his life as a witcher. The two talked and prayed for some time before the Great Beggar sent Merten to the Hidden Chapel in Murky Delve to continue his pilgrimage. Five days later, on the 19th, the locals got drunk and, tired of the Great Beggar's antics, robbed the temple and walled him up in his quarters, where he was left to starve to death.
However, due to how he died, his spirit couldn't move on and so he turned into the Great Beggar's Ghost, a spirit that attacked anything that came within proximity. Geralt can lay his spirit to rest permanently by placing his remains in a sarcophagus in the antechamber.