Can a Fathomless Warlock's Tentacle of the Deeps help flank?

Question

The Fathomless Warlock has a feature called Tentacle of the Deeps, which allows them summon a 10-foot long spectral tentacle (exact wording below). It can be moved and make attacks,but is spectral and doesn't necessarily have a physical presence. However, it can affect events around it by dealing damage, partially blocking both physical and magical damage types at later levels (described in Guardian Coil below), and moving around. This raises the question of if it is considered a creature for the purpose of flanking. While it's never explicitly stated to be one, it shares many traits of one, and as evidenced by many creatures like ghosts, being corporeal isn't necessarily a requirement to be considered a creature.

Tentacle of the Deeps:

At 1st level, you can magically summon a spectral tentacle that strikes at your foes. As a bonus action, you create a 10-foot-long tentacle at a point you can see within 60 feet of you. The tentacle lasts for 1 minute or until you use this feature to create another tentacle. When you create the tentacle, you can make a melee spell attack against one creature within 10 feet of it. On a hit, the target takes 1d8 cold damage, and its speed is reduced by 10 feet until the start of your next turn. When you reach 10th level in this class, the damage increases to 2d8. As a bonus action on your turn, you can move the tentacle up to 30 feet and repeat the attack . You can summon the tentacle a number of times equal to your proficiency bonus, and you regain all expended uses when you finish a long rest.

Guardian Coil:

At 6th level, your Tentacle of the Deeps can defend you and others, interposing itself between them and harm. When you or a creature you can see takes damage while within 10 feet of the tentacle, you can use your reaction to choose one of those creatures and reduce the damage to that creature by 1d8. When you reach 10th level in this class, the damage reduced by the tentacle increases to 2d8.

Answer 1

No, it would not help you with flanking.

Flanking requires status as a creature, as you mentioned, and nothing about the two abilities linked mention that this is a creature.

We can use this Answer for an example of 'what is a creature'. The ability does not hit at any of these.

The Tentacle is not an independent, or controlled, entity. It has no health, armor class, statistics. The tentacle isn't even the actual being to make the attacks that it makes, you are mechanically making spell attacks against anything within 10 feet of the tentacle it isn't actually 'making the hit' you are. The tentacle lacks any ability to take actions, reactions, move itself, and other features that you would see on a creature for 5e.

The issue of it being 'spectral' isn't a problem. A ghost is incorporeal but a ghost also has everything I just mentioned. Things like Bigby's hand, Spiritual weapon, and other spells also do not create creatures.

Answer 2

The tentacles are not "Creatures"

The rules for flanking require the target to be flanked by two creatures:

When a creature and at least one of its allies are adjacent to an enemy and on opposite sides or corners of the enemy’s space, they flank that enemy, and each of them has advantage on melee attack rolls against that enemy.

It seems clear enough that "one of its allies" is also intended to be a creature. This is confirmed in the next sentence of the flanking rules:

When in doubt about whether two creatures flank an enemy on a grid, trace an imaginary line between the centers of the creatures’ spaces.

We can compare the tentacle to the weapon summoned by the spell spiritual weapon:

You create a floating, spectral weapon within range that lasts for the duration or until you cast this spell again. When you cast the spell, you can make a melee spell attack against a creature within 5 feet of the weapon. On a hit, the target takes force damage equal to 1d8 + your spellcasting ability modifier.

As a bonus action on your turn, you can move the weapon up to 20 feet and repeat the attack against a creature within 5 feet of it.

The 1st level version of the tentacle functions nearly identically to a Cleric's spiritual weapon, which is obviously not a creature, and the 6th level feature just gives the warlock a new way to use the feature.

This same question is asked about spiritual weapon, with the same conclusion, here: Using the optional flanking rule, would a Spiritual Weapon on the other side of an enemy allow a character to benefit from flanking?

It should be mentioned that "creature" is never given a precise definition in the rules, so there is some room for a DM to rule that the tentacles are creatures, however that ruling seems to be mostly unsupported by what we do have in the rules. Waxeagle gives a good analysis of what constitutes a creature in the answer to this question: What is the definition of "creature" and is it used consistently?

Allowing the tentacles to create advantage through flanking makes the feature extremely powerful.

Allowing the tentacle to count as an ally for the purposes of flanking is essentially free advantage for all of your melee fighters. You can move the tentacle 30 feet each turn with your bonus action, so that it can maintain a flanking position at all times, and you still have your action to do whatever you need to do. The kobold's "Pack Tactics" feature is often criticized as being too powerful for a racial trait as it can often give the kobold character advantage on most of its attacks. Allowing the tentacle to give advantage through flanking will allow all melee characters advantage on most of their attacks. It takes a known overpowered feature (Pack Tactics) and dispenses its benefits to every character.