This isn't my first post of magic items for 5e, though it is the first since the release of the DMG. As I've discussed before, I love some of the new levers we see in 5e magic item design. The magic items of the DMG are, by majority, the traditional range of D&D items, like you'd expect from the core book. They cover their bases, but I don't think they take a lot of risks here. Hoard of the Dragon Queen and Rise of Tiamat start to push a few more boundaries here, with magic items that custom-fit the plotline that rewards them. This immediately endows them with a greater sense of lore and connection.
The salient point here is that I have a lot of fun creating magic items for my own campaign. It's much more common to see magic items with two or three unique properties, and relatively rare to see a lot of pure-math benefits. In posting a series of new magic items (this being my thirdsuch post), I don't expect that anyone reading this would use any of these items as written. After all, much of my audience are DMs, looking for the pleasure of sharing with their players the content they dreamed up. Borrowing someone else's ideas might take all the joy out of it for them. Instead, my hope is that they'll steal a piece here and there, or will see something that is a jumping-off point for their own innovations.
The Bone Mask of the Naga
Rare, requires attunement
In the course of exploring a necromancer's tomb, my PCs discovered sizable bone fragments, as well as a strange blue-black substance that was an adhesive when used on two pieces of bone. With only a little effort, they fit the shards back together, forming a massive snake skull. It turns out that it's the skull of a bone naga, enchanted as a protection against other bone nagas. (In this way I also committed myself to using additional bone nagas in the future.)
This mask is made from the skull of a bone naga, and grants the wearer some of a bone naga's traits when the mask is attuned.
Darkvision, 60 ft
Resistance against poison
Each time you encounter another bone naga, you may choose one of the following:
Ignore the bone naga's damage immunity to poison
Become invisible to the bone naga until you take an action that would normally break invisibility
Ignore the bone naga's condition immunity to charmed
Deal 2d8 extra damage the first time you deal damage to the naga
The Horn of the Long Night
Rare, requires attunement
In the distant past, when the world was young and the Veil between the world we know and the Land of the Dead was not yet fully woven, the Archfey warred with the Powers that rule the Land of the Dead - be they gods or greater things still. Now the Archfey are known for holding the will of mortals in thrall, inspiring fear or twisted adulation. The undead that are anything greater than thralls, however, are immune to such influence. Without the chiefest of their weapons, then, the Archfey were nearly powerless. They stole a pack of Hell's own hounds and bred them with the finest, fleetest hounds bred by mortals. The get of that breeding they ensorcelled with further power and granted to the Master of the Wild Hunt. He turned his hounds to the relentless pursuit of those who belonged in the Land of the Dead and had slipped its power, through magic.
This horn is fashioned as an echo of the one that the Master of the Wild Hunt bore. It is a glossy black, chased with runes and knotwork of silver. For its bearer, it conjures not the hounds themselves, but their terrifying phantasms.
While attuned to this horn, any time an undead would be charmed or frightened by an effect you cause but is immune, it is harried instead, by phantasmal hounds of the Wild Hunt. (This assumes the undead was a valid target for the charmed or frightened effect in the first place, and the undead receives a normal saving throw against the initial charmed or frightened effect, if appropriate.)
Harried is a condition that causes the following:
The creature must pass a Wisdom save against the original spell DC to leave its current space. On a failure it may still move, but if it chooses to do so, it takes 1d6 damage (scaling up with level as per a cantrip - 5th, 2d6, 11th, 3d6, 17th 4d6). Once the creature passes the saving throw or takes damage, the harried condition ends.
The creature suffers disadvantage on all Concentration, Intelligence, and Wisdom checks (not saving throws in general, though).
This is an unsubtle workaround, as magic items go. Fey-pact warlocks face a bad problem when it comes to many kinds of undead, immune as they are to the charmed and frightened conditions. The harried condition won't stick around long, though you have the option of continuing to use effects that would cause the charmed or frightened effects. After all, you're spending a spell slot to do the same damage as a cantrip of your level. (AoE charms/fears are highly potent with the Horn of the Long Night, admittedly... you know, like the Fey Presence power.)
Unique, attunement possible
Vardevain was a powerful necromancer of the distant past, who came upon a kingdom in which the incorporeal undead ruled, even in the light of day, over the living. Though possibly motivated by his own hunger for power, he bound the spectral undead, and in so doing freed the living from their dark and tyrannical masters. Vardevain's Rod was one of the key implements of the great rites he performed to defeat the undead. It is made of a dark metal, with a twisted cage on one end that holds a mote of sickly green light.
While unattuned, the rod allows its wielder to add 1d8 damage to any spell that deals necrotic or cold damage, once per long rest.
Attunement requires the binding of an incorporeal undead to the rod, to act as a conduit of power. Once it is attuned, it has the following properties:
+1 bonus to spell attack rolls, spell DC rolls, and cantrip damage rolls.
Instead of adding 1d8 to the damage of a spell once per long rest, it has 8 charges. Each charge adds 1d8 damage to a spell that deals necrotic or cold damage. The rod regains 1d4 charges every day at midnight.
It grants advantage on any Arcana checks to perform necromatic rites (if needed).
It possesses a fragment of Vardevain's intellect and may converse with the wielder at its discretion, though it does not possess Ego or an ability to compel the wielder.
Blade of the Deep Waters
Rare, requires attunement
This short sword bears a wave-like pattern in its metal, indicating both a connection to elemental water, and extraordinary talent in its forging. When attuned, it bears the following properties.
+1 bonus to attack, damage, and saving throw DCs triggered by weapon attacks
The weapon changes its properties whenever it is used to defeat an amphibious creature, an elemental connected to water, or a creature native to the sea. It does not change if you defeat a second creature of the same category as it already reflects. If a creature could reasonably belong to one or more of those groups, determine the effect randomly at the time the creature is defeated.
Amphibious: While carried on your person, the weapon increases the time you can hold your breath underwater tenfold. If carried by a creature that cannot breathe air naturally, the creature can hold its breath in air ten times as long.
Elemental Water creature: You can enter a hostile creature's space and remain there. You can move through a space as narrow as 1 inch without squeezing.
Sea native: Once per short rest or long rest, you gain a hunter shark's blood frenzy ability, gaining advantage on melee attack rolls against any creature that doesn't have all its hit points.
The shifting abilities of this item, often without the wielder's intent (until they learn what to expect, and plan accordingly) is inspired by the lens focused on the unknown Unearthly item in Metroplexity. I recently got sucked back into Metroplexity, and its unearthly items are a wonderfully rich idea mine for surprising, weird, and evocative items that expand the setting in light-touch kinds of ways. They're essentially the game's paid content and DLC; at $5+ a pop, they have to offer a lot of interesting content to feel like a good thank-you gift - a standard I think Kinak meets and exceeds.