Base bonus to XP = 10%
MAX bonus to XP = 20%

Unique soul core x 0.2% (Max with 50 soul cores)

Soul Core drop chance ~20%
Soul Prism drop chance ~10%

Soul Core and Soul Prism drop only from Fiendish creatures
Tip: Find Fiend spell "exiva moe res" Tells you which direction the nearest fiendish creature is.