Formal:
* Unprecedented: This is a strong word that implies a complete lack of any previous examples.
* Novel: This emphasizes the newness and originality of the action.
* Pioneering: This suggests a groundbreaking or trailblazing effort.
* Innovative: This focuses on the creative and inventive nature of the action.
Informal:
* First time ever: This is a simple and straightforward way to express the idea.
* Brand new: This emphasizes the freshness and lack of precedent.
* Groundbreaking: This suggests that the action is significant and will have a lasting impact.
* Trailblazing: This is similar to "groundbreaking" but suggests more of a pioneering spirit.
Other options:
* Unheard of: This suggests that the action is completely unknown and unexpected.
* Without precedent: This is a formal way of saying "never been done before."
* First of its kind: This emphasizes the uniqueness and originality of the action.
The best word or phrase to use will depend on the context of your writing.