Binding of Souls

Binding of Souls costs {x} less to cast, where X is
the amount of life you have lost this turn.
Whenever you lose life, you may sacrifice this permanent, if you do, gain that much life instead
(If you were to gain more than 400 life this way,
gain 400 life instead).
If Binding of Souls would enter the graveyard
from anywhere, exile it instead.