Dead Count Shapeshifter

Dragon Revelation Command Spells. The unique Command Spells engraved on Sieg. They can also be used as normal Command Spells, but there was absolutely no need for Sieg to force Astolfo to do anything with a Command Spell, so he used them all to turn himself into Siegfried. To explain it simply, they basically let Sieg summon Siegfried every time he uses one. The catalyst is Siegfried's living heart. However, while Siegfried's body could withstand the dragon blood within it, Sieg's body can't. That is reason that the spots on Sieg's hand where the Command Spells disappeared from turned black. Even after he used them all up, power Sieg obtained from Frankenstein's “Blasted Tree: Lightning Branch of Crucifixion” managed to barely maintain his human form, but that too was used up when Sieg used this Noble Phantasm at the end.

Fate/Apocrypha material: Fate/Apocrypha Encylopedia