Rathma's Shield

Rathma's Shield is a  passive skill, unlocked at level 70.

In-game
This skill does not actually provide immunity to damage: instead, it prevents Life from being drained, so Crowd Control effects from enemy attacks will still occur.

As of patch 2.7.0, it also has a 60 seconds cooldown.

However, it does prevent the Blood skills from draining the Necromancer's Life for its duration:
 * Blood Nova, Blood Spear, Life Support
 * Thy Flesh Sustained, Possession, Blood Rush (except Hemostasis rune), Simulacrum
 * Early Grave
 * Final Embrace, Blood Lance, Oblation