Phantasms are monsters with "Blighted" in front of their name and are similar to Providence's Shade in his final phase in appearance.
They only spawn in Monsoon Difficulty and are a force to be reckoned with.
Phantasms do not seem to spawn when the Honor Artifact is active. Apparently, they aren't treated as Elite Monsters as described by the artifact.
Phantasms are a step above their elite variations. They are invisible in nature and only are visible when committing an action, such as attacking, rolling, or teleporting. They have capabilities similar to that are given by items like leaving a trail of fire behind them and having their attacks explode, much like Fireman's Boots and Brilliant Behemoth . These Phantasms have increased health pools, movement speed, and damage making them hit harder than anything in the game with their damage scaled by ~1000%.
Note: Damage scaling may be incorrect with the separate possibility of them dealing a percentage of your maximum HP.
When fighting a Phantasm they will have a health bar. However, like any other monster, Phantasms are susceptible to Telescopic Sight.
Additionally, since they are treated as 'elite enemies', 56 Leaf Clover may proc upon killing them thus granting an extra item to the player. It appears that, since they have multiple Elite properties (Blazing, Frenzied, etc.) they can drop 2 or 3 items.