The Chains of Heaven. One of Gilgamesh’s favorite Noble Phantasms.
As the chain that captured the Bull of Heaven, which had driven Uruk into a seven year famine, it is Gilgamesh’s most trusted Noble Phantasm—even more so than Ea. Its power is to “restrain the gods.”
It is a Noble Phantasm which grows more sturdy in proportion to the target’s divinity, making it one of the few anti-god weapons.
Consequently, for Berserker (Heracles), Enkidu was an even more troublesome Noble Phantasm than Excalibur.
On the other hand, for Servants with no divinity like Saber, Assassin, or Archer, it is nothing more than a normal, sturdy chain.

Fate/side material: Fate Encyclopedia

One of Gilgamesh’s noble phantasms. Said to be chains used to subdue the divine beast “Bull of Heaven.” Has the ability to restraint targets. The more divine the opponent, the greater its power.

Translator’s Notes
  1. ^ Chains of Heaven.
Fate/Zero Animation Guide II: Glossary of the holy grail war II