To understand the firebird, it’s essential to first dig into a Grishaverse concept called “amplifiers.” If Grisha are a bit like magicians with particular areas of expertise, think of those amplifiers like amulets that enhance their natural abilities. Most Grisha’s amplifiers come from the remains of a specific animal — one that, in the case of the Netflix series, the Grisha feels a certain connection to. In season 1, Alina dreamed constantly of a white stag, and its antlers turned out to be her amplifier. In season 2, Tamar (Anna Leong Brophy) tells Alina her own amplifier was a shark, whose tooth she still wears in her ear.
According to Tamar, amplifiers can only be made if the animal they come from is killed by the Grisha, a rule Alina refuses to accept. The Sun Summoner tries to put this theory to the test with the sea whip, a water-dwelling dragon that she begins dreaming about after the stag has died. Unlike other Grisha, who only ever have one amplifier their whole lives, the Sun Summoner dreams of three. “I think they are to be connected through me so I can then destroy the Fold,” Alina tells the others, and since she’s a prophesied hero, there’s no reason not to believe the rules of magic will bend for her.
Unfortunately, Alina does end up having to kill the sea whip with her magic when it lunges for Mal. Still, Alina has one more chance to prove her theory right: the firebird. In Grishaverse lore, the firebird is an imposing and highly respected creature that’s pretty much the same as a phoenix. That’s how Alina envisions it, too, until Mal uncovers the truth: that he’s the firebird.
Source : https://www.slashfilm.com/1232467/shadow-and-bones-hunt-for-the-firebird-ends-with-a-twist/