The Specter is a skin for the Sentinel class in Tribes: Ascend. It can be unlocked for 975 Gold.
"Most think Specters are a myth, somewhere to place blame when a stray shot finds a victim. The Twenty prefer it that way." In-game description.
Trivia
The Specter is the first semi-transparent skin in Tribes: Ascend. It wears glowing green goggles which give it a mystical look, and has green jetpack boost as default.