In the quest to discover what “There can be only one” means, I tried sacrificing all fifth tier towers at once, sad to say I don't think anything happened. Based on the
BTD6 bloonarius攻略 第6回目マップ:In The Loop | KMのゲームブログ
why there can only be one : r/btd6
There can be only one :) : r/btd6
There Can Be Only One (Special Mission) | Bloons Wiki | Fandom
Magic Knowledge There Can Only Be One - KnowledgeWalls
There Can Be Only One (Monkey Knowledge Point) | Bloons Wiki | Fandom
How to unlock the "There Can Be Only One" Dark True Sun God! (Bloons TD 6 Tower Showcase Bonus) - YouTube