The Duel on the Astral Plane was a confrontation between Doctor Strange and the Zealot Lucian in the Astral Dimension.
While defending the New York Sanctum from the Zealots, Stephen Strange used the Rotunda of Gateways to trap his right-hand man Lucian in a rainforest. After trapping and restraining the separatist faction's leader Kaecilius, Strange learned about the Zealot's plan to merge the Earth with the Dark Dimension as well as the Ancient One's attempts to prolong her life by drawing power from the Dark Dimension.
Unbeknownst to Strange, Lucian managed to find his way back to the Sanctum and crept up behind the distracted sorcerer. Just as Strange noticed that his Sling Ring was missing, Lucian stabbed him with a Space Shard. The two engaged in hand-to-hand combat only for Lucian to gain the upper hand.
At the last minute, the Cloak of Levitation wrapped itself around Lucian's face and proceeded to suffocate him, allowing Strange to find his Sling Ring and conjure a portal leading to Metro-General Hospital.