Shadow of Moil is a 4th-level necromancy spell with the following description (XGtE, p. 164):
Flame-like shadows wreathe your body until the spell ends, causing you to become heavily obscured to others. The shadows turn dim light within 10 feet of you into darkness, and bright light in the same area to dim light.
Until the spell ends, you have resistance to radiant damage. In addition, whenever a creature within 10 feet of you hits you with an attack, the shadows lash out at that creature, dealing it 2d8 necrotic damage.
The core rules state about Truesight (PHB p. 185):
A creature with truesight can, out to a specific range, see in normal and magical darkness, see invisible creatures and objects, automatically detect visual illusions and succeed on saving throws against them, and perceive the original form of a shapechanger or a creature that is transformed by magic. Furthermore, the monster can see into the Ethereal Plane.
Would a character under the effect of Shadow of Moil still have advantage against a creature with Truesight? Essentially, are "flame-like shadows" darkness, if spells do exactly what they say they do?
Best Answer
Yes, Shadow of Moil heavily obscures against Truesight
Truesight can see through darkness and illusions, but in cannot see through magically conjured physical objects. A spell such as Fog Cloud, for instance, would be effective against a creature with Truesight, because the fog is conjured into physical existence rather than just cast as an illusion.
The question then is whether the "flame like shadows" are physical objects capable of obstructing line of sight, or whether they are an illusion or magical darkness.
I would argue that the "flame like shadows" are unlikely to be illusions because the spell does not specify them to be illusory. In addition, they are dissimilar from the effects illusion spells in that they are capable of dealing necrotic damage and most damage dealing illusions, with the exception of Illusory Dragon, deal psychic damage. The flame like shadows are not a form of darkness because they heavily obscure even against darkvision unlike the surrounding area of darkness that they create. This interpretation is supported by the fact that the "Flame-like shadows" are first described as heavily obscuring the user and then are described as decreasing the light levels around you. Whereas the spell Darkness specifies that darkvision does not help Shadow of Moil does not implying that darkvision helps against its darkness but not against the heavily obscured condition.
Since the shadows are neither illusion nor darkness, they are effective against Truesight.