It shows up when you cleared the first 9 steps.
- Added Limited Doll on Step 2 and 4
- Added 1000x Star Dia on Step 5
- Removed prizes in Step 1, so we only have 8 steps now.
I don't think sega mention a specific date for the gacha ends, but I've seen the expiration date for Arena Coin (Red) via inventory, it saids August 6.
Also:
※ The Step resets when all prizes are dawn in the Final Step
※ When a new Arena Gacha is added the Step will Reset to the Initial Step
Source: https://bumped.org/idola/idola-weekly-vol-21/
3 times here:
1st - 1 fragment
2nd - 0 fragment
3rd - 3 fragments
I think it's doable without Stella, cleared mine with these characters:
Law: Nadia, Abeille, Flami, Sasha
Chaos: Anna, Walter, Astraea, Barnabas