At some point in Shin Megami Tensei IV: Apocalypse, you will be given a quest called “u have a guest”. By the terrible spelling from the quest’s details page, it should be obvious who the quest was created by. This guide helps you complete the quest.
Quest details :
You hae a vistor, Nanashi! Thee said they want Yamata-no-Orochi, but… I have 0 clue y there here. Duder’s kinda scary, so hury up!
How To Complete “u have a guest” Quest in SMT IV: Apocalypse
How to get Orochi by demon fusion
You should see Orochi as one of the demons you can fuse. If you do not see it, you should try to recruit more demons to your team and fuse more demons to get better demons. You may have a better chance of having one of the demons you need to fuse to get Orochi.
Alternatively, if you already had Orochi, you can summon him again from the Demon Compendium in exchange for Macca.
Completing the quest after getting Orochi
Once you have Orochi, use the Yoyogi terminal to transport yourself near to the Fairy Forest. After entering the Fairy Forest, you should be at the Southern Pond Bridge. Keep moving forward and you should enter the Kakuun Path area. Continue moving forward until you see a cutscene and you will see some dialogue.
The reward is a Compendium Ticket that lets you summon a demon from the Demon Compendium for free. In addition, you also get an upgrade to your Ame-no-habakiri sword (Ame-no-murakumo). Its slash power bumps up from 190 to 296.