From LSWiki

Joining

To join the Cult of Sacrifice, you must find the secret location and say the secret password. It appears that Nerius only speaks Caladan - more testing there. An additional requirement seems to be that worship of other gods is forbidden.

Nerius says, "The Dark One requires your full spiritual devotion, why are even here if not to sacrifice everything, Dicia? "  

When all requirements are met -

You say, "I wish to sacrifice myself to the Dark One" in beautifully-accented Caladan.

    You feel a malicious presence enter your soul and enrich you with demonic powers.

benefits

Joining the Cult of Sacrifice provides you with demonic abilities via demonic blood.