If you want to tame it listen to the tip that said you can "force tame it in the arena but you will be stuck there" just spawn it using commands:

GMSummon EndBoss_Character_C 150 (Tamed)

Summon EndBoss_Character_C (Wild)

GMSummon EndBoss_Character_Hard_C 150 (Tamed Alpha Overseer)

Summon EndBoss_Character_Hard_C (Wild Alpha Overseer)