Night Pilgrims by Chelsea Quinn Yarbro
In Night Pilgrims, Saint-Germain is living in a monastery in Egypt when he is hired to guide a group of pilgrims to underground churches in southern Egypt. The vampire finds a companion in a lovely widow who later fears that her dalliance with the Count will prevent her from reaching Heaven. The pilgrims begin to fall prey to the trials of travel in the Holy Lands; some see visions and hear the word of God; others are seduced by desires for riches and power. A visit to the Chapel of the Holy Grail brings many quarrels to a head; Saint-Germain must use all his diplomacy and a good deal of his strength to keep the pilgrims from slaughtering one another.