Artifacts are any exceptionally powerful items, namely items of level 7 or higher.

Crown of Kings +5 Increases the Strength, Intelligence, and Agility of the Hero by 5 when worn.

Level Range Area of Effect Allowed Targets Other 8 N/A Unit Self

Mask of Death While wearing this mask, a Hero will recover hit points equal to 50% of the attack damage dealt to an enemy unit.

Level Range Area of Effect Allowed Targets Other 8 N/A Unit Self See Below

Claws of Attack +15 Increases the attack damage of the Hero by 15 when worn.

Level Range Area of Effect Allowed Targets Other 7 N/A Unit Self

Ring of Protection +5 Increases the armor of the Hero by 5 when worn.

Level Range Area of Effect Allowed Targets Other 7 N/A Unit Self

Orb of Frost Adds 6 bonus cold damage to the attack of a Hero when carried. The Hero's attacks also become ranged when attacking air and slow the movement speed and attack rate of the enemy for 3 seconds.

Level Range Area of Effect Allowed Targets Other 7 N/A Unit Air, Ground, Ward 3 second duration, See Below

Tome of Power Gives the Hero an experience level.

Level Range Area of Effect Allowed Targets Other 8 N/A N/A Self Power-up Item

Mask of Death and Orb of Frost
If you have multiple Orbs, the damage bonus stacks, but the effect does not. Only one effect is used which is the highest in the inventory. If you have manually (not autocast) activated attack modifier ability such as Searing Arrow, it will be used instead of the orb. However, if Autocast is selected, the Orb will override the ability.

The same is the case with the Mask of Death ( = Orb)

Hero Items Index