This is a mission type event. It lasts from June 3 to June 16 and requires having cleared Solomon.
New Servants:
4* Sakamoto Ryouma (Rider, welfare)
3* Okada Izo (Assassin, limited time)
5* Okita Alter (Alter Ego, limited time)
Gamepress's guide.
Infographic for quick reference. Put together by Soudumpling.