When you're done looking for the four fragments of the weapon, go to the tree and a cut-scene will commence. When it ends, Senua will gain access to a new weapon - Grymr. You will be able to test the sword immediately. A couple of enemies will appear - note that they often coat themselves in Darkness, which requires Focus to deal any damage to them.
When the fight ends, go along the walls of human arms. After a moment, you will reach a great arena and, as you probably expect, a bunch of enemies will try to kill you. Watch out - there will be shieldsmen, giants and the regular opponents. After you manage to explain to them you're not getting killed today, proceed forward, straight into another fight.
A new type of enemy will appear. They will be armed with two, one-handed axes. They sort of remind one of the earlier bosses. That's the strongest type of enemy in the game, right here. Very quick, using strong combos and able to throw the axes. Sometimes they shroud themselves in darkness - you need to use focus then.
When you defeat an enemy, a real flood of enemies will come. The game sure wants to make up for all those fragments when you had no weapon. Fight with them and go forward. You will eventually reach a path forward.
Walk on the giant's arm.
Run pass the zombies and climb on the giant. Use it's arm to get to the head - you will see that the giant has the face of Senua's Mother. You will be moved to the beach that we've visited some time ago.