Today is the Harrowhelm, by Victor Selby. I felt was powerful enough to assign Artifact status to. I don't have the DMG yet, so I'm not sure the level of power matches.
This is a psionic item. For the Mind Blast power, I simply copied the power of the same name from the Mind Flayer. The Innate Spellcasting (psionic) are the same powers of the Githyanki Knight.
I do like psionics in my game. I don't have any in currently, but some are in place. I plan on using the Psionicist and Psionic Warrior that Khaalis posted over at Enworld.
Artifact, requires attunement
You must be a non-psionic character that can wear metallic armor to attune the Harrowhelm.
When first found, this helm appears to be any other sort of normal steel helmet; it radiates magic, however, and faint runes may be seen on the inside rim of the helm.
You gain the ability to generate a mind blast up to three times per day, but only once in any single round. The Harrowhelm emits psychic energy in a 60-foot cone. Each creature in that area must succeed on a DC 15 Intelligence saving throw or take 22 (4d8+4) psychic damage and be stunned for 1 minute. A creature can repeat the saving throw at the end of each of its turns, ending the effect on itself on a success.
You are also immune to psychic damage except for mind blast, but the helm grants Advantage on saving throws against the mind blast.
You also gain Innate Spellcasting (Psionics). The Harrowhelm’s spell save DC is 15, +5 to hit with spell attacks. You can innately cast the following spells, requiring no components.
At will: mage hand (the hand is invisible)
3/day each: jump, misty step, nondetection (self only),
1/day each: plane shift, telekinesis
The Harrowhelm has two disadvantages. First, it attracts the attention of all psionic creatures within a half-mile radius as soon as it is put on (whether or not any of its powers are immediately used). Though it does not necessarily anger such creatures, it will be easy for these beings to locate and track the helm wearer as long as they stay within this range; they need only concentrate mentally to do this.
The second disadvantage manifests itself when a psionic character puts on the helm. The unfortunate wearer will immediately be attacked by the mind blast effect, and has Disadvantage on saves against the mind blast.