The appearance of Veldora Tempest "Tensei Shitara Slime datta ken", Veldora Becomes Human
Veldora Tempest in the Tensura anime has occurred in Episode 12 Season 2. After a long wait, Rimuru finally managed to turn Veldora into a Human and free him from Infinite Prison.
Rimuru managed to Evolve into a Demon Lord after massacring the kingdom of Falmuth. After the massacre, Rimuru summoned 3 High-Level Devils and one of them was Noir (Diablo). The three devils were ordered by Rimuru to chase after Razen (Wizard of the Falmuth Kingdom), Now while RImuru started the Harvest Festival for the Awakening of a New Demon King.
Skill had an important role in the awakening and liberation of Veldora Tempest. He replaces Rimuru to continue the Harvest Festival. This was done because Rimuru was in "Sleep" Mode, while the Harvest Festival could not be postponed. In the Process, the Great Sage's Skill Evolved into Raphael at the expense of the Degenerate Skill.
As we know, That the Great Sage and Rimuru did an analysis of Veldora's Seal (It was done since Veldora entered Rimuru's Stomach). Well after the Great Sage Evolved into Raphael, Then Raphael's analytical ability becomes better and faster. It made Raphael “Faster” in analyzing Veldora's Seal.
After Rimuru woke up from "Sleep", and the Harvest Festival was over. Raphael said that the Analysis of Veldora's Seal had been carried out successfully.
Veldora Becomes Human
The analysis of Veldora's Infinite Prison has been completed. Then Raphael said that the Infinite Prison Confinement Period was over and Veldora was ready to be Freed. Hearing all that, Rimuru was very “Happy” because he was able to meet and fulfill his promise to Veldora. Rimuru made a clone of himself to be made into a container by Veldora, Then Veldora possessed the clone's body and in the end Veldora was freed.
Veldora / Veldora's appearance as Human is in Episode 12 Season 2 of Tensura. So to see Veldora in action, we have to wait until next July 2021 (Continuation of S2 Tensura).