You can unlock the gates to Helheim once you've managed to pickup keys from both bosses. You'll be subjected to a long cut-scene that sees you land on a bridge leading to the land of the dead. A small group of enemies will appear - fight them one by one and make sure to get rid of all opponents.
You'll have to face a few opponents on the bridge
Start making your way to the tower. You'll stunned by a new opponent moment you've cross the threshold. Your enemy, a giant armed with an axe, is very similar to a Surt. Get back to your feet and dodge to avoid being attacked. Continue dodging and engage your opponent from his sides. Fortunately, the giant is very slow and you shouldn't have any problems staying away from the range of his axe.
You'll have to face additional opponents after defeating the giant. Defeat them, and continue through the wooden door to pick up runes from the opposite wall. Turn right and pick up a story rune. Continue down to the ship and recreate the stairs by employing focus mode.
Use them to climb to the upper floor and use a ladder to access the tower. Activate your focus and find the first rune, then descend down the ladder and use the ledge nearby to move left - there is a hidden face of Senua's mother at the end of the ledge.
Return to the main hall and once again take a look at the sign hanging on the chains to discover a new rune. Descend down the ladder to pick-up the story rune and unlock the door. There is a door with additional runes before you - unlock it and continue down the bridge to begin a cut-scene. It turns out that Senua is not yet ready to fight the Goddess.