Akechi is a boss that can be accessed via party window under boss tab.

How to Enter Akechi Mitsuhide Boss

You must finish all of the asura crisis quests in order to unlock akechi as a weekly boss. if the GO button is not available to you that means you havent fully finished the prequest.

Here is a list of each of the quests that are required in order to unlock Akechi.

(Lv.200) [The Asura Crisis] Back to Momijigaoka

(Lv.200) [The Asura Crisis] A Royal Summons

(Lv.200) [The Asura Crisis] A Wandering Swordsman

(Lv.200) [The Asura Crisis] Ayame’s Investigation

(Lv.200) [The Asura Crisis] Ambush the Oda

Head to the Hidden Mana Shrine you can visit here by talking to Ayame.

(Lv.200) [The Asura Crisis] Oda’s Fortress

(Lv.200) [The Asura Crisis] Heavy is the Head

(Lv.200) [The Asura Crisis] An Elemental Problem

(Lv.200) [The Asura Crisis] Recover the Five Elements

(Lv.200) [The Asura Crisis] The Princess’s Speech

(Lv.200) [The Asura Crisis] An Unexpected Conversation

Head to the portal in the right in Momijagaoka and speak to Sakuno

(Lv.200) [The Asura Crisis] Seeking Sakuno

(Lv.200) [The Asura Crisis] The Revelation

(Lv.200) [The Asura Crisis] Find The Princess

(Lv.200) [The Asura Crisis] War’s End

(Lv.200) [The Asura Crisis] Crisis Averted

Head to the portal in the right in Momijagaoka and speak to Sakuno

(Lv.200) [The Asura Crisis] A Secret Promise

(Lv.200) [The Asura Crisis] Sacrifice’s Reward (the last quest to complete)

For more information on Akechi feel free to check digitaltq‘s quest guide on the boss