Aunt Talitha's Cross was the cross of Talitha Unwin. She gave it to Roland Deschain after his visit to River Crossing.
Later, it was given to John Cullum to act as a sigul so that Moses Carver would believe his story and help found the Tet Corporation. After Cullum's death, it was passed on to Carver, who returned it to Roland when he visited the headquarter of Tet in New York City in 1999.
As promised, Roland laid it at the base of the Dark Tower.