In Hellblade Senua’s Sacrifice, after Senua activates the tree by trying to pull the sword from the tree, the 3 shards around the tree activates. Senua must enter each shard and finish the trial before she can journey onwards into Hel.
This is Baldr’s Trial.
So first, enter and then use the face mask.
Then go between past and present to solve the various puzzles in Baldr’s Trial. You will need to repair the stairs first.
Find the S sign.
Find the Up Arrow Sign.