Doctrin and Covenants 29:36-41.
Shall I tell you the law of God in regard to the African
race? If the white man who belongs to the chosen seed mixes
his blood with the seed of Cain, the penalty, under the law
of God, is death on the spot.
This will always be so. Cain slew his brother... and the
Lord put a mark upon him, which is the flat nose and black
That curse will remain upon them, and they can never hold
the Preisthood or share in it untill all other descendants
of Adam have received the promises and enjoyed the blessings
of the Priesthood and keys therepf.
When all other children of Adam have had the privelege of
receiving the Priesthood, and of comming into the kingdom of
God, and of being redeemed from the four quarters of the earth,
and have received their resurrection from the dead, then will
be time enough to remove the curse from Cain and his posterity."
All of this however is the least of the problems with the Mormon
religion. If you knew what happened once you got in so far that you
could not turn back (This is after taking the oath of death...) you
would laugh, then you would probably say, "That's insane... How could
people believe this."