Ominous Dummies are blocks used to upgrade nearby hostile mobs.
Ominous Dummies can be found by looting an Archives.
When placed, an Ominous Dummy can be damaged like a mob, and broken by destroying the block portion of it underneath.
Whenever an Ominous Dummy takes damage, a red indicator will pop up, displaying the damage amount. The number will appear visually larger for higher amounts of damage.
Ominous Dummies will affect mobs which spawn up to 64 blocks away. Specifically, Ominous Dummies will buff all undead (Zombie, Bogged, Wither, etc) and raider (Pillager, Witch, Ravager, etc) mobs which spawn nearby.
Buffed mobs will spawn with random armor if possible (including mobs that can wear animal armor such as Zombie Horse and Zombie Nautilus). This armor will appear trimmed by the Sentry Smithing Template using a Lapis Lazuli material.
Buffed mobs also receive permanent Bad Omen, Strength II, and Resistance II status effects. Buffed undead mobs are additionally granted permanent Fire Resistance, and buffed raiders are given Speed I.
Mobs buffed by an Ominous Dummy will not drop items on death, except for mounts (Zombie Horse, etc) and mobs which usually have exactly one guaranteed single drop item (Wither, Evoker, etc).
See also: Target Dummy