Iceheart is a Monster Set (Helm and Shoulder) available in light, medium or heavy armor. It drops in the Iceheart (unique) style.
The 1-piece set bonus adds 833 Spell Critical.
2-piece set bonus: When you deal Critical Damage, you have a 20% chance to gain a damage shield that absorbs 8600 damage for 6 seconds. While the damage shield holds, you deal 770 Cold Damage to all enemies within 5 meters of you every 1 second. This effect can occur once every 6 seconds.
Iceheart is a good option for Magicka-based "tanky" melee players, or Magicka-based dungeon tanks who use ice staves, since Cold Damage can proc the Chilled status effect, which gives a Minor Maim debuff, reducing the enemy's damage by 15%. You also get a damage shield when the set bonus procs.
Magicka DPS players can also use 1 piece of Iceheart to boost their spell crit, and combine it with another Monster item of their preference.
The Shoulder can be acquired by using an Undaunted key on Gilirion the Redbeard's chest. The Shoulder type, weight and trait are RNG based and can be from any dungeon this NPC gives Pledges for.
The Helm can be acquired from Direfrost Keep which is part of the base game and located in Eastmarch. The Helm is guaranteed to drop from the final boss in veteran mode only. The weight and trait are RNG based.
You can mix-and-match Monster Set shoulders and helms from different sets and add two 1-piece bonuses to your build, or wear a complete set to take advantage of the full 2-piece set bonus.