Benimaru/Chronology

Orc Invasion of the Ogre Village
Benimaru was introduced in the Forest Disturbance Arc as the Young Master of the Ogre Tribe. The village was attacked by 10,000 Orcs led by the Orc Lord Geld and Majin Footman.

Meeting with Rimuru
Having his entire tribe eradicated, Benimaru and his 5 remaining tribe members ran away with great regret, only to encounter a Goblin Rider patrol and later Rimuru. They fought due to the Ogre group's frustrations and a misunderstanding caused by Rimuru wearing the Anti-Magic Mask, but it got resolved thanks to Benimaru's little sister's efforts and since no one died, Rimuru healed everyone with his potions and they all went into Rimuru's Goblin village. where a Feast was about to be held.

After some rest, Rimuru learned about Benimaru's and his companions' circumstances and asked them if they were willing to become his subordinates, even if it was temporary since they too would need to fight these orcs, it would be better to join forces. Benimaru asked for some time to think about it and walked away, only to meet his childhood friend and rival stating that it wasn't a bad idea after looking at the state of the goblin village Rimuru ruled over. After some frustrating reminiscence, Benimaru finally decided to accept and finally, as a symbol of Rimuru's rulership, granted the six of them names: Benimaru, Shuna, Souei, Shion, Hakurou, and Kurobe, and thus they all evolved from Ogres into Kijin.

Against the Orcs
Some days passed and Benimaru, Hakurou and Souei trained together to get used to their newfound powers.