Does the reach increase of the Eldritch Claw tattoo stack with Fangs of the Fire Snake?

Question

The Eldritch Claw tattoo released in Tasha's Cauldron of Everything includes the Eldritch Maul ability which grants the following benefit:

For the duration, each of your melee attacks with a weapon or an unarmed strike can reach a target up to 15 feet away from you, as inky tendrils launch toward the target.

The Fangs of the Fire Snake elemental discipline also grants the following benefit:

Your reach with your unarmed strikes increases by 10 feet for that action, as well as the rest of the turn.

After activating Eldritch Maul and using Fangs of the Fire Snake what is the reach of my unarmed strikes? Would it be 25 feet or would it be 15 ft?

Answer 1

Reach (the noun) is a game term.

The rules for melee attacks say:

Used in hand-to-hand combat, a melee attack allows you to attack a foe within your reach. A melee attack typically uses a handheld weapon such as a sword, a warhammer, or an axe. A typical monster makes a melee attack when it strikes with its claws, horns, teeth, tentacles, or other body part. A few spells also involve making a melee attack.

Most creatures have a 5-foot reach and can thus attack targets within 5 feet of them when making a melee attack. Certain creatures (typically those larger than Medium) have melee attacks with a greater reach than 5 feet, as noted in their descriptions.

Every creature has a statistic called reach that is the range at which they may make melee attacks on a target.

Reach (the verb) is not a game term.

Eldritch Maul says:

For the duration, each of your melee attacks with a weapon or an unarmed strike can reach [verb] a target up to 15 feet away from you, as inky tendrils launch toward the target.

This does not increase your Reach (the creature statistic). "Reach" here is used as a verb to assign a particular usable range to the Eldritch Maul ability. It does not add anything to the creature statistic Reach.

In contrast, Fangs of the Fire Snake adds 10 feet to your Reach (the creature statistic):

Your reach [noun] with your unarmed strikes increases by 10 feet for that action

This actually increases your reach. If we have a natural reach of 5 feet, and we use Fangs of the Fire Snake, now our reach is 15 feet.

Since Eldritch Maul uses a bonus action, it will always come before Fangs of the Fire Snake, if they are to be in effect at the same time. Let's break this down.

1. Bonus action Eldritch Maul. We can now reach targets up to 15 feet away with unarmed strikes. Our reach (creature statistic) is still 5 feet.

2. Attack action, spend a ki point to activate Fangs of the Fire Snake. This adds 10 feet to our reach (creature statistic) when we use unarmed strikes, bringing our reach to 15 feet for unarmed strikes.

Answer 2

Eldritch Maul does not stack with other reach increases.

If you started with a reach of 5 ft and then activated 2 different effects that each increase your range by 10 ft, your total reach would indeed be 25 ft. However, the reach granted by Eldritch Maul is worded in such a way that it does not stack with other reach increases. Instead, it simply says that your melee attacks can reach targets 15 ft away from you. If your reach is already 15 ft or greater, this has no effect at all. If your reach is less than 15 ft, it becomes 15 ft. This is analogous to how the Barkskin spell affects your AC:

You touch a willing creature. Until the spell ends, the target's skin has a rough, bark-like appearance, and the target's AC can't be less than 16, regardless of what kind of armor it is wearing.

Just like Barkskin doesn't stack with other AC increases (such as donning a shield), Eldritch Maul does not stack with other reach increases, but rather imposes a minimum reach on all your attacks.

For another example of a similar non-stacking effect, see this question about increasing your natural strength score while wearing a Belt of Giant Strength.