Deathblock

Other sources:

 * Green Steel accessories
 * Yellow augment
 * Cannith Crafting
 * Embraced the Void, in the Pale Master enhancement tree (needs level 20)
 * Embraced the Void, in the Pale Master enhancement tree (needs level 20)

Monsters:

 * Epic Ward grants Deathblock to Orange Named enemies.
 * Certain Monster Champions are granted Deathblock:
 * Tier 1 - Stygian
 * Tier 2 - Flame Born, Hero's Soul, Light Bearer, Shadow Marked
 * Tier 3 - Mark of Law