Secret Boss
In order to fight the secret boss of the game, you must first obtain the party member after beating the volcano. Following that, fly to the cave right next to the left of the beginning of the game. The secret boss is at the end of the hallway and It requires training, because it will kill every party member on it's fifth or sixth turn.
The sequel to the award winning Hellblade: Senua’s Sacrifice, Senua returns in a brutal journey of survival through the myth and torment of Viking Iceland. Intent on saving those who have fallen victim to the horrors of tyranny, Senua faces a battle of overcoming the darkness within and without.