What item can the magic mask be used against?  Answer the above question based on the context below:  A "skybike", a one-man, open-cockpit flying machine, attacks Dogen. Dogen shoots it down and finds one of Syn's crystals on the pilot's body. Carved into the crystal is a symbol of a dead tree. Dogen finds a murdered prospector, whose young daughter Dhyana saw him killed by Baal, Jared Syn's half-cyborg son. Baal sprayed the man with a green liquid that caused a nightmare dream-state, in which Syn appeared and executed him with a crystal. Dogen convinces Dhyana to help him find Syn. Dhyana takes Dogen to Zax, who identifies the crystal as a lifeforce storage device. Dhyana tells them about the ancient Cyclopians who once used such devices and says the only power against it is a magic mask located in their lost city. Zax affirms this and directs Dogen to find a prospector named Rhodes in the nearby mining town of Zhor. Dogen and Dhyana are blocked by vehicles driven by nomads commanded by Baal, who sprays Dogen with the green liquid, paralyzing him. Dhyana drives them off and cares for Dogen, who in the dream world finds Syn and Baal looming over him. Syn fails to pull Dogen away from Dhyana: their will is too strong. Dogen awakes, but Dhyana is suddenly teleported away. A summoned monster appears in her place and fires electric bolts at him. Dhyana simultaneously faces Syn in his lair. Dogen shorts-out the creature, and it vanishes. Dogen arrives in Zhor and finds Rhodes, a washed-up soldier, in a bar. Rhodes denies the lost city's existence and refuses to get involved. Dogen leaves and comes upon a group of miners beating a captured nomad soldier. Dogen assists him, and the miners turn hostile. Dogen is out-gunned until Rhodes helps him defeat the miners.
Answer:
the crystal