Can a Tricksy fey from the Summon Fey spell place a cube of darkness partially on an enemy, so allies can still target what’s visible at advantage?

Question

The summon fey spell (TCoE, p. 112) summons a fey spirit marked by a chosen mood, which determines the secondary effect of its Fey Step bonus action (after teleporting 30 feet). For the Tricksy mood, the following effect occurs after it teleports:

The fey can fill a 5-foot cube within 5 feet of it with magical darkness, which lasts until the end of its next turn.

Is it possible for the fey to place the 5-ft. cube of darkness over the upper body (mid-torso and up) of an enemy, effectively making any attacker an unseen attacker?

Per the rules on unseen attackers:

When a creature can't see you, you have advantage on attack rolls against it.

If the lower portion of the enemy can still be seen (mid-torso and below), is a character able to target what they can see of the enemy and attack with advantage?

Answer 1

This requires a DM ruling

The core issue here is whether different parts of the same creature can be considered to be in different lighting conditions. Unfortunately, the rules for vision and light don't seem to provide any clear guidance here, as far as I can tell.

As DM, you might decide that for the sake of simplicity, each creature is considered to be under a single lighting condition, i.e. every creature is either in darkness (i.e. heavily obscured by lighting conditions), dim light (lightly obscured by lighting), or bright light (not obscured), and the mechanical effects of that lighting condition apply equally to all parts of the creature's body. Alternatively, you might decide that the described use of the 5-foot cube of darkness is a clever one and allow the player to use it to cover only the top half of a creature's body.

Personally, I would recommend the first ruling, because of the rules for control of space:

A creature's space is the area in feet that it effectively controls in combat, not an expression of its physical dimensions. A typical Medium creature isn't 5 feet wide, for example, but it does control a space that wide.

If you created a 5-foot cube of darkness that covers a creature from the waist up, the creature could potentially just crouch down in response and see under the cube, while remaining within their 5-foot space. Hence, a static cube floating at waist height might not be effective in blinding the creature, whereas a static cube at ground level would cover the creature's entire space, which unambiguously shrouds the creature's whole body to darkness until they leave that space.