Then equip the horn and blow it at night in order to summon the Cultist Elder (Boss).
After defeating him, you can loot the White Cultist's Staff from him, equip said staff, upon interacting with the Cultists Altar at night, you can start the ritual with the Ancient Skull in your inventory.
When the ritual begins, the skull will then be placed on the altar, with a health bar slowly depleting when the White Cultist's Staff is close enough to it (equipped).
Before it's completely depleted, some of the pillars surrounding the altar will start to lit. After about 10 seconds of lit they will explode, damaging you (a lot) if you stand too close to the pillars.
So remember to move to the farther side of the Altar or even go behind other unlit pillars to reduce the possible damage. Using Bandages and White Flower Potions is also recommeded to survive the ritual.
Once the skull's health is depleted, the ritual will be over and the skull will shine with a bright light. You can then collect the Purified Ancient Skull and return it to Lenvell.
Talk to Lenvell and select "The Cleansed Skull", he will tell you the following:
The Cleansed Skull
You've done it! I knew you had it in you. You've done a great deed by removing this curse. The world is now a safer place.
Now that the skull is no longer cursed, I believe we can use it to make something that would be of great value to you.
Then you can ask him about "Using the skull".
You will receive another two options:
Ancient Skull Staff
Do you remember those cultists that I told you about? Well they carry some powerful artifacts. According to my studies, the staves that they carry have some interesting properties.
They sacrifice some of their own blood in order to imbue the staves with extra strength. Don't ask me how it works, because I have no idea. If you can obtain one of these staves, and affix the cleansed skull on top, I would be willing to bet it would make a very powerful weapon.
Take note that you should not use the white staff that you used to cleanse the skull for this purpose. The white staff would subdue the power still contained in the skull, which would make for a very inept weapon indeed.
I don't know if you are the hunter type, but if you are, I have a suggestion for you. Take two pristine deer pelts, and fashion them together in the shape of a large pouch, adorn the front of the outside with the purified ancient skull. I think you will be quite pleased with the result.