Search for the Diamond Staff
Search for the Diamond Staff, published June 2013
I think this is the last mention of the Blood of Lathander before BG3, which would make it most likely the place it was stolen by sharran thieves before the events of that game.
Lathander’s Open Hand:
"This small shrine is a converted barn with stone walls and a wooden roof. A bell hangs above the front door in what was formerly a hayloft. Inside is a chapel with an altar dedicated to Lathander, the god of the dawn (known to most of his followers as Amaunator, the god of the sun). A small back room serves as the living quarters for the resident priest. A fist-sized chunk of amber carved in the shape of a heart and shining with a golden light is magically suspended fifteen feet above the altar. This relic is known as the Blood of Lathander. If removed from the shrine, it teleports back to this spot one minute later unless it is borne by a dedicated follower of Lathander.
The priest of the shrine is Sister Tressa, a matronly human female and one of the few Lathanderites remaining in the Dalelands. She has five potions of healing hidden in her room, and is willing to sell them to characters who seem good-hearted (50 gp per potion)" (9)