The Spiked Armor can be found in Sword Coast Adventurer’s Guide, page 121.
This suit of armor is reinforced with a magically enhanced metal alloy called mizzium, which is made in Izzet foundries. While you're wearing the armor, any critical hit against you becomes a normal hit. In addition, when you are subjected to a magical effect that allows you to make a Strength or Constitution saving throw to take only half damage, you instead take no damage if you succeed on the saving throw.
Spiked armor is a rare type of medium armor made by dwarves. It consists of a leather coat and leggings covered with spikes that are usually made of metal.
The wearer has Disadvantage on Dexterity (Stealth) checks.