The “Hollow Mission” are quests you’ll be taking on to acquire special skills, weapons, and more upon completion.
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Before heading out to take on a Hollow Mission, you’ll be able to group up with your buddies and prepare for the quest. While doing this in the offline mode, you’ll be advancing through the story as you complete missions. In online mode, upon clearing the Hollow Mission, you’ll be sent back to the base.
The Material Gathering system is another special feature that you can take advantage of in the Hollow Area. By meeting certain conditions for various objectives, the game will grant you Hollow Points, which can then be used to acquire special skills and equipment.
There will also be some other special unlocks you can get by completing various lists.
While Aincrad does indeed have 100 floors, in Sword Art Online: Hollow Fragment, you’ll be starting out from a town on the 76th floor, as you clear fields, dungeons, and bosses to work your way to the top. However, there will be many powerful bosses that will require the proper preparations before taking on. You’ll be able to check out all kinds of information on the bosses before taking them on, so studying these monsters and their habits will be just as crucial as keeping your gear up to scratch.
Similar to the show and novel, there will also be a “Last Attack” feature. There will be certain weapons that can only be obtained by getting in the last hit on bosses, so you might also want to watch out for that, before a pesky NPC ends up stealing the glory!