After obtaining the Sunfire, return to Altair. The Dreadnought will be hovering over the town. Use the Snowcraft to reach the Dreadnought's landing pad. This is a one-way trip, so ensure you are fully prepared. The Dreadnought is a linear dungeon, but it's heavily guarded by Imperial Soldiers, Commanders, and powerful mechanical enemies. Your objective is to reach the engine room and use the Sunfire. Be cautious of traps and hidden passages. There is no boss fight within the Dreadnought itself, but the sheer number of encounters can be draining. Once the Sunfire is used, the Dreadnought will be destroyed, and you'll escape just in time. This marks a major victory for the rebellion.
