A Dark Ritual

A Dark Ritual is the fifth quest in the Main Quest line, and continues immediately after finishing the previous quest, A Cure For The Curse.

Walkthrough
Upon completing A Cure For The Curse, the quest begins immediately.

You need to first loot the Cultist's Horn from the Cultists, which spawn near Cultists Altars located on Uninhabited Islands.

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:Then you can ask him about "Using the skull".

You will receive another two options:This describes the crafting recipes for two different variants of the Ancient Skull Staff, one made with the Cultist's Staff, and one with the White Cultist's Staff.This describes the crafting recipe for the Ancient Skull Quiver.

Trivia

 * Obtaining the Ancient Skull can be performed beforehand more than once by saving, storaging & loading, so it is possible to get more than one Purified Ancient Skull (version 1.9.5).
 * If properly positioned and moved, standing on the Altar while performing the ritual can avoid all the explosive damage from the lit pillars, regardless of the seemingly irreverent behavior.