Is this homebrew whip blade balanced?

Question

One of my players has asked if he can have his character use a whip blade (i.e., a sword that transforms into a whiplike chain to deal damage at a distance). See here for a picture. Based on my discussions with him, he would like to start with this weapon.

Whip Blade
Martial Melee Weapon

Cost: 50 gp
Weight: 3 lb
Damage: 1d6 slashing
Properties: Special

When using a Whip Blade, you may interact with it to toggle between solid blade form and whip form. In blade form, the weapon has the Finesse property. In whip form, the weapon has the Reach property, but attack and damage rolls using this weapon must be made using your Dexterity modifier.

My hope is to create a weapon that is balanced (i.e., neither strictly better nor strictly worse) relative to the other martial weapons in the table in the PHB (pg. 149). What changes need to be made for this to be the case?

Answer 1

The whip blade is unbalanced for dexterous characters and pointless for strong characters.

(I am assuming that the interaction to toggle forms is completely free because no action is mentioned in the whip blade's description)

Toggling forms

In most cases, a character can change their reach once during their turn by dropping a weapon and unsheathing another weapon that has more or less reach.
By comparison, the whip blade can toggle its form to change the reach as many times as necessary per turn without the hassle of dropping stuff onto the ground. I think that the toggle ability is decent in the hands of a tactical player, but not game changing.

Dexterous characters (dex ≥ str)

The whip form's lack of finesse is inconsequential because the whip form uses dex anyway.
The whip form is strictly better than whips because of the extra damage.
The whip form is at least on par with rapiers and scimitars. However, if the dexterous character has the dual wielder feat then the whip form is strictly better than scimitars.
Moreover, the whip form is strictly stronger than the blade form too, so the toggle ability is useless outside of some really rare scenarios where having reach is a negative.
Overall, I would almost always use the blade whip with a dexterous character over any other melee weapon.

The exception are rogues because they need the finesse property for sneak attacks. However, rogues are not proficient with whip blades, so it's not worth analyzing.

Strong characters (str > dex)

The finesse property is meaningless to strong characters and so the blade form is strictly worse than all one-handed martial melee weapons except for the whip and the lance, and generally worse than all martial melee weapons.
The whip form is strictly worse than whips when you consider the reduced chance to hit from using dex instead of str.
Overall, I would almost never use the whip blade with a strong character.

My recommendation

Just flavor a regular whip to be a whip blade. When the enemy is within 5' describe the attacks as belonging to a solid blade, else describe the attacks as belonging to a whip-like sword. Guaranteed balanced.

Once the game progresses to the point that magic weapons are available, put something like this in the loot.

Whip Blade
Weapon(whip), rare

You have a +1 bonus to attack and damage rolls made with this magic whip. When you make an attack with it during your turn, you can forgo its reach property to deal an extra 1d4 slashing damage.

Answer 2

Yes the whip-blade is balanced against other martial weapons

The cost/weight are negligible, they don't impact gameplay in any meaningful way, so I'm just going to ignore them.

The 2 most similar weapons to this are the Short Sword and the Whip, so lets compare them.

Blade Form vs Short Sword

Both are 1d6 finesse weapons. The only differences are the damage type and that the Blade Form lacks the light property, which means no two-weapon fighting.

Whip Form vs Whip

The Whip is a 1d4 slashing, finesse, reach weapon. The Whip Form is a 1d6 slashing, reach weapon which adds +Dex to hit and damage. The tradeoff is upgrading the damage die to a d6 while losing the option for high Str characters. 1d4 -> 1d6 is +1 expected damage, which doesn't really matter in the long-term (base weapon damage die is one of the least significant factors in determining damage past the first few levels).

Whip-Blade vs Carrying 2+ weapons

Depending on what is required to change weapon forms determines how good the Whip-Blade is, but it certainly isn't overwhelmingly better than using a Short Sword and a Whip.

You lose the option to use two-weapon fighting in the Blade Form, and you gain +1 damage in Whip Form. If changing forms is an object interaction or Bonus Action than the Whip-Blade is slightly better than carrying a Whip and one (or more) Short Swords. If changing forms requires an Action than I would say you would be better off with multiple weapons, especially if those multiple weapons are a Rapier/Dart (both finesse, 1d8 and 1d4 average to the same as two 1d6s).

Answer 3

I'd say it's balanced, possibly even weak, assuming that what you mean by "interact" is an object interaction, of which you get a free one with a move or attack action. This presents ways to do something similar within the rules, though not the same exact effect.

Use two weapons

You could switch between a finesse weapon and a reach weapon: the finesse weapon could be a rapier for d8 damage, but swapping it out for a reach weapon requires you to attack using strength or choose a whip, which then limits you to d4 damage. Considering that the average damage (d8 vs d4) is about the same as a d6, then the trade between the weapons is fair and it could be effectively treated as having a single d6 weapon.

Throw your melee weapon

Alternatively, you could homebrew a weapon chain that attaches the weapon to your wrist, allowing you to throw it and then retrieve it. The damage would be that of an improvised weapon (a d4), and the free interaction could be to retrieve the weapon, allowing two attacks in the same turn (though the weapon would then be unready until the next turn) or one attack every turn and the weapon would be constantly ready. The down side here is that an improvised thrown weapon still used strength for its attack and damage. So again, we have a melee damage of up to d8, but a reach/ranged damage of d4.

RAI

The fly in the ointment, however, is that it seems that the weapons list goes out of its way limit one-handed dex based reach/range attacks to d4 - whip is reach, dagger is finesse thrown, dart is ranged. Everything else requires two hands (bows and crossbows) or is strength based (polearms for reach and spears/axes/javelins for thrown). This may be a conscious decision on the part of the designers or it might just be random chance. As such, any homebrewing in this area should be carefully considered. It doesn't seem to me that a d6 finesse reach/thrown is utterly broken, but they seem to be curiously omitted, so it seems reasonable that someone considered them to be.

Your proposal

Considering the options available to do this without improvisation, the increased cost and restrictions on the reach weapon effectively make this a weak polearm for dex characters. The cost is quite high compared to the other options above, but as they do not achieve the exact result you desire, it may be worth it.

But don't fool yourself - ultimately, what you have ended up with is a whip that does d6 damage for 50G. The restriction requiring the use of Dex is not really a restriction, as any Strength based character using martial weapons has much better options, though they are burdened with the heavy and two-handed features. And the intended recipient, likely the only person in your campaign to ever use the weapon, suffers no negative impact from the "requires dex" restriction. if this is okay with you, then just simplify the weapon to be d6, finesse, reach and be done with it.

Playtest

Whatever you decide, make it clear to the player that the weapon is an experiment, and if you decide that it is overpowered (or underpowered, though this seems unlikely), you reserve the right to tweak it as needed.