The Musphelheim crucible is an arena based side activity in God of War Ragnarok that will put you up against some enemies with varying restrictions to add some challenge to the event.
In order to take part in the crucible events, you will need to have found the second Musphelheim seed piece, this will activate the gateway to allow you travel there.

From Santa Monica Studio comes the sequel to the critically acclaimed God of War (2018). Fimbulwinter is well underway. Kratos and Atreus must journey to each of the Nine Realms in search of answers as Asgardian forces prepare for a prophesied battle that will end the world. Along the way they will explore stunning, mythical landscapes, and face fearsome enemies in the form of Norse gods and monsters. The threat…