The Gilbert Rings protect people from being killed by supernatural beings or causes, as long as the rings are worn. ... John gave his own ring to Isobel, who then gave it to Alaric. John then took possession of Grayson's ring after the latter's death. There are only two known Gilbert Rings in existence.

Where did Alaric get his ring?

When Damon brought this up, Alaric informed him that he got his ring from Isobel, and asked Damon if he thought John knew Isobel as well.

Why does the ring stop working on Alaric?

Klaus won't stop until he gets those coffins back — he's not done with Jeremy. At home, Alaric awoke but started spitting up blood. My theory: The ring didn't work at full strength because while Alaric was killed by a hybrid, that hybrid was driving a human car.