Magic Resistance: The Adamantite Horror has advantage on saving throws against spells and other magical effects
Magic Weapons: The Adamantite horror's weapon attacks are magical, and it has a +3 to melee attack & damage rolls (Included).
Shatter Vulnerability: If the Adamantite Horror fails its saving throw against the Shatter spell, it becomes blinded for 1d4+1 rounds.
Linked Mind(Ex): All clockwork horrors within 10 miles of a gold, platinum, or adamantine horror are in constant communication. If one is aware of a particular danger, they all are.
Siege Monster: The Adamantite horror deals double damage to objects and structures.
Multiattack: The Adamantite Horror makes two Razor saw attacks.
Razor Saw: Melee Weapon Attack: +12 to hit, reach 5 ft., 1 target. Hit: 19 (2d10 + 8) slashing damage.
Nightmare Stick(Recharge 5-6): The Adamantite Horror casts Disintegrate, targeting a creature it can see (DC 19).
Description
The Adamantite horror is not just rare but unique. The father of all Clockwork Horrors, the Adamantite horror leads the entire race it created, and only one exists on the Material Plane. At least, only one per reality..
Suggestion: the iron golem has a CR of 16, and that's just iron. If this thing is to live up to its name you should raise its CR, a lot, especially with those stats holy. My one other note is, is it supposed to be small? I might be missing something, but I feel like it should be a bit bigger, especially if it's supposed to be a siege monster.