When the word 'boss' is mentioned, if you are a gamer who was immersed in the world of games, the first thing that would appear in your mind would be the long huge HP bar that fills a part of the screen.
However, if you are a person who sought the path of 'git-gud,' then you would know every boss monster has the potential to be reborn or enter a new phase.
Either their HP bar would be filled, or their attacks would take a new form. And, the way they do it would mostly be a cutscene.
We, as a player, would see a cutscene where the boss monster would either power up, scream, roar, or speak some words.
This was the same with the Time Guardian.
Considering the game Legacy of Shadows: The Hunter's Destiny was inspired by the souls-like games, it was no wonder that was the case.
The second phase would start with a loud roar, where the monster would release a scream filled with time attribute mana.