An ally of unknown components said to come from the ancient Umbra. Some demons choose to drink it while others just pour it over themselves, letting the alloy create a natural defense.
Stackstone is a possible upgrade for Alloy of the Ancients as this is a powerful card, and doubling its effects is powerful provided you can handle the increased cost.
Eternalstone is an occassionally great upgrade for Alloy of the Ancients if you have a small enough deck to play Alloy of the Ancients multiple times in a battle or are confident you will be able to also upgrade it with Keepstone, and can handle the increased cost. In larger decks, Stackstone or even no upgrade is better.