Is there a reason this homebrew spell wouldn't work with the Attack action?

Question

The spell in question:

Sendak’s Lightning Lash

2nd-level evocation

Casting Time: 1 action
Range: 10 feet
Components: V S
Duration: Concentration, up to 1 minute.

You bring your hands together, then draw out and strike a target creature with a whiplike arc of lightning. Make a melee spell attack against that creature. On a hit, the target takes 2d8 lightning damage. The arc remains in your hand for the duration or until you let go of it, which ends the spell. The arc acts as a melee weapon you are proficient with, which deals 2d4 lightning damage on hit and has the finesse, light, and reach properties.

At Higher Levels. When you cast this spell using a spell slot of 3rd level or higher, the initial damage increases by 1d8 and the weapon damage increases by 1d4 for each slot level above 2nd.

Being able to use this weapon with an Attack action (and by extension, Opportunity Attack, Extra Attack and, via twinning, Two Weapon Fighting) is the intended behavior, unlike something like Flame Blade. Does the wording work?

Answer 1

Yes this gives you a usable weapon

Once the arc is conjured with the Cast a Spell action, you can attack with it like any other weapon.

Here is the relevant text from the Introduction on Weapons in the Player's Handbook:

A melee weapon is used to attack a target within 5 feet of you.

You should, however, specify that is a melee weapon. Otherwise it is ambiguous.

As written it will not work with Twinned Spell the way you intend

Twinned Spell makes you target a second creature. You do not cast the spell multiple times. In this case, the initial whiplike arc hits two creatures, but is still only one arc.

When you cast a spell that targets only one creature and doesn't have a range of self, you can spend a number of sorcery points equal to the spell's level to target a second creature in range with the same spell

This can be fixed by saying something like, "Whether you hit or miss, an arc of lightning remain in either of your hands for each target of this spell. These arcs last for the duration..." I would add this as a new paragraph since it is a separate part of the spell.

Answer 2

Sendak’s Lightning Lash should not be Twinnable

For several reasons:

The range of similar spells is Self

Sendak’s Lightning Lash gives the caster an effect and attacks another creature. In practice, the spell has two targets. There are many other spells that behave in this way, and they all have a range of self. See for example Produce Flame, Flame Blade, Shadow Blade, and Vampiric Touch. The design intent is that a range of self overrides the other range, so Sendak’s Lightning Lash should also have a range of self and should not be twinnable.

The behavior of weapons is incompatible with twinning

Sendak’s Lightning Lash explicitly describes that the arc weapon "deals 2d4 lightning damage on [a] hit", which means that everything the caster hits with the arc weapon is also a target of the Sendak’s Lightning Lash. Likewise, anyone who suffers a Dragon's Breath is also a target of Dragon's Breath. Therefore, Sendak’s Lightning Lash can target more than one creature during its duration by making melee weapon attacks against differing creatures. This means that it can't be twinned, just like Bigby's Hand, Telekinesis, and Maximilian's Earthen Grasp.

Twinned spells should not affect one creature twice.

The primary reasons why Twinned Spell has the targeting restrictions that it has, is to ensure that a twinned spell cannot be redirrected to effect one creature twice. Moreover, it is the logical conclusion of the rulings I linked above for Dragon's Breath and Maximilian's Earthen Grasp. Your intended behavior for a twinned Sendak’s Lightning Lash goes against this intent in two distinct ways:

1. The caster receives a double buff in the form of two arc weapons.
2. A single target can be attacked by both arc weapons.

Conclusion

It may be possible to very carefully and artificially word Sendak’s Lightning Lash to get around these problems, but then you are effectively designing a spell around a metamagic and your spell will forever stick out like a sore thumb among every other sorcerer spell.

If you really want Sendak’s Lightning Lash to support two weapon fighting, I recommend just giving the spell that functionality, instead of relying on some janky interaction with Twinned Spell.

Answer 3

This seems to work as you intend it to work. By describing it as "a weapon", it'll follow all the normal rules for weapons.

Note that Valor Bards (and possibly others) are going to have a field day with their new [many]d4 weapon and access to Extra Attack and Bonus Action attacks. You might want to reconsider letting the weapon damage with level.