Warning: Spoilers ahead.
Following The Rising of the Shield Hero Season 2 - Episode 9, this ep details what was happening to Raphtalia while Naofumi’s party was separated from her.
She’s taken to a prison called the Prison of Exhaustion, that drains your energy so you can’t escape. She will be held until the bastard Kyo comes to get her.
What luck! She’s put in the same exact cell as the Vassal Heroes, L’arc, Therese, and Glass. Kyo’s says he’s a genius but he stupidly allowed this to happen.
L’arc and the others are surprised to see Raphtalia in their world, and smaller as well. But they can tell from her personality that it’s really her. They exchange notes, and find out Raphtalia was stuck in the Infinite Labyrinth. Where they helped free their dear comrade, Kizuna. So Glass vows to help return Raphtalia back to Naofumi.
Raphtalia is able to use illusion magic to conceal herself from the guard. They steal his talisman to become more powerful and blast out of the prison!
The party heads for Sikul to reunite with Naofumi. They plan to help Raphtalia to level up as much as possible on the way.
With Therese’s crystals, they’re able to buy a change of clothes to disguise themselves and a sword for Raphtalia to level up.
She does level up a bit and grows, too. Her body goes through growing pains.
The next day, L’arc party discovers that there is an event where the country will try to find the new Katana Vassal Hero. To prove worthy of being the Katana Hero, the person must remove the Katana from a stone.
So they go to check out the event. They discover that the bastard Kazuki is the candidate who will try to take the Katana. But as he tries, the Katana teleports out of his grasp and into Raphtalia’s hands for some unknown reason! As if she was the chosen one.
Of course, this pisses off Kazuki. He orders the guards to hunt down Raphtalia. Raphtalia doesn’t want to cause trouble for L’arc’s crew, so she runs away on her own.
A little later, Raphtalia starts growing to adult size. It’s painful for her. Luckily a handmaiden for the temple idolizing the Katana Hero finds her and helps her. She gives Raphtalia clothes meant for the Katana Hero.
Kazuki tracks her down, surprised to find an adult. He doesn’t know that this is the small demihuman he met already.
Anyways, Raphtalia is able to beat the shit out of him, and even kill his guardian beast copies - Byakko tigers. She leaves and tells him to warn his master Kyo that the Shield Hero is coming for revenge.
Then Raphtalia faces the real Byakko. She’s out of energy and would have died, if not for the timely appearance of Naofumi’s party! They save her. And Naofumi isn’t angry at Raphtalia for what she did.