The Hardcore Revival Altar is a mod that I've put together for use in multiplayer hardcore SMP servers.

It allows players to make a sacrifice in order to bring a player back from the dead.

NEEDS: A mod that adds the ability to get player heads. I use Just Player Heads by Serilum (here)

HOW IT WORKS:

You must place this block such that it is three block above a redstone block. There should be two free blocks in-between the redstone block and the altar. You'll know it worked because lightning will strike 4 places around the block, then 4 bolts will strike the block's immediate surroundings.

Then, you place the dead player underneath, in spectator mode, and have your sacrifice walk onto the altar. They will die, and the dead player will be revived! Both the altar and redstone block will be removed in the process, and moss will take the place of the redstone.

AS OF 1.1.0: If you have been brought back from the dead, you will not be able to sacrifice yourself to bring another back.

CRAFTING RECIPE:

Two amethyst shards, two totems of undying, four diamonds, and one player head gets you one revival altar. NOTE: The altar is single-use, so craft wisely!

The recipe for the revival altar. Four diamonds, one in each corner. Two amethyst shards, on top and bottom middle. Two totems of undying, on the middle sides, and one player head, in the center, to get one Revival Altar