This page of our guide to TES V: Skyrim contains a detailed walkthrough of The Ancestral Worship side quest, which can be unlocked at Hillgrund's Tomb. The hero has to help Golldir and defeat a mini-boss - Vals Veran the necromancer.
New main objective: Defeat Vals Veran
New optional objective: Protect Golldir
Open the world map and head to Hillgrund's Tomb found in the eastern part of Skyrim (screen above). If you haven't yet discovered it, it would be best to begin at Windhelm and from there head south-west. After reaching the destination, open the door and approach Golldir nearby the entrance. The man will ask you for help in eliminating the necromancer Vals Veran, who's desecrating the bodies of his ancestors. You should of course offer your help, learning that Golldir's aunt has already went inside. Note that Golldir will join your party as an independent follower, so you won't need to give up on your current one.
Wait for Golldir to open the passage and use the door leading inside Hillgrund's Tomb. Begin exploring this area and look out for the Draugrs that you will soon come across. I'd recommend being very careful, as you most of all will come across the stronger kinds, i.e. Draugr Wights, Draugr Scourges and Draugr Deathlords. Adapt your tactic depending on whether you're fighting a warrior, archer or mage. Continue onwards and you will reach a room with the corpse of Agna and a locked door (screen above). Golldir will suggest using an alternative passage.
Open the western door and head onwards. On your way you will of course have to fight some more Draugrs, but that should be too much of a surprise. A rather linear path will lead you to your destination. After reaching the southern room, hear out Golldir's hints and find the chain beside which there's a bear symbol (screen above). Use the eastern passage. Stop after finding the door leading to the main chamber and save your game just in case.
Open the iron door and start attacking Vals Veran. Unfortunately the necromancer will teleport after receiving a couple hits (screen above), forcing you and your team to fight Draugrs. Focus mainly on those who will try to kill Golldir. His premature death won't end the whole quest, though it's worth to keep him alive to the very end.
New main objective: Speak with Golldir
As you fight the Draugrs, keep an eye on the compass and start attacking Vals Veran when only he returns to the battlefield (screen above). You need to alternate between attacking the necromancer and the Draugrs until Vals Veran eventually dies. If Golldir is still alive, speak to him to receive gold and Hillgrund's Chest Key. In case Golldir died, search his body and take the key. Apart from that, search the necromancer's body and take, inter alia, the Hillgrund's Tomb Crypt Key. Use the key to open the northern door and the other one on the treasure chest. In the end return to the tomb exit, look for Golldir and consider inviting him to travel with you further, this time as a full time party member.