The Hematic Key

The Hematic Key is a special that occurs in every game in Temple of the Firstborn Level 1 in  of Diablo III. Completing it is the only way to access Level 2 and get to Vidian.

Deep into the Temple, the Nephalem will come across a chamber with empty pools and lines on the floor, full of praying Blood Cultists. The Cultists will then attack the Nephalem.

With each killed Cultist, pools will fill with blood, making it form runes and circles.

When nearly all Cultists are dead, the door to Level 2 will unlock, and end the event.