Location: Mages Guild in the Town of Riverhold
Speak to Jo'Rasma the alchemist at the Riverhold Mages Guild. She is researching daedric alchemy and wants you to fetch 12 orc ancestor skulls from the Daedric shrine of Malauch (Malacath), west of Orcrest.
Travel to the shrine to fetch the skulls. Jo'Rasma wants 12 of the enchanted ones and some are duds. You will know which ones are enchanted when you pick one up because something appears when you do!
Several orc pilgrims have gathered at the shrine and guard the skulls. The priest Graaush Bonekeeper is away in Orcrest during the course of his quest, so he will not accidentally die if things go south.
Taking a skull counts as stealing. Its best to sneak and grab one when no one is looking. If you are spotted the pilgrims might get nasty and attack you. If you are not a member of the Thieves Guild they will also snatch back a taken skull and add it to their own inventory.
You might then have to pickpocket it back from them, or kill them, or have a monster kill them.
Once you have 12 of the magical skulls (the dud ones don't count!), return to Jo'Rasma and she will reward you with 5 daedric blood potions and 5 daedric magicka potions. They come in those snazzy daedric bottles.
She will also sell more to you in the future, and after the quest is complete respawn in her shop's inventory.