In the middle of the lake (the subzone is named, likewise, Blood Statue) there is a huge statue (possibly representing Lilith), half-immersed into a pool of blood. Around it will be dozens of sacrificial offerings and praying Blood Cultists.
Upon killing every single Cultist, the Nephalem will be tasked with defacing and destroying the statue. Daivin, if in party, will disagree, but the event proceeds regardless. A single hit will make the statue's face fall off, reducing it to rubble.
This act will summon forth a horde of Blood Vermin. Killing them all ends the event.