Keystone Signet

Acquisition
Signet of Capture
 * Prophecies
 * Rune Ethercrash (Spearhead Peak)
 * Nightfall
 * Qwytzylkak (Vehtendi Valley only during The Killing Joke)

Trivia

 * A keystone is a vital piece in a physical or conceptual construction, such as a stone arch or an ecosystem.