#The screenplay says it’s “a crackling rope of supernatural light”.

“In a split-second NEWT pulls the cocoon from his pocket and releases it at GRAVES. The Swooping Evil soars around him, shielding NEWT and the Aurors from GRAVES’S spells, and giving NEWT time to raise his wand.

 

With a sense that he’s been holding this one back, he slashes it through the air: out flies a crackling rope of supernatural light that wraps itself around GRAVES like a whip. GRAVES tries to hold it off as it tightens but staggers, struggles and falls to his knees, dropping his wand.”
_{^{- Fantastic Beasts and Where to Find Them: The Original Screenplay}}

“Be quiet!’ shouted Hermione, but it was too late: Umbridge pointed her wand at Magorian and screamed, ‘Incarcerous!’

 

Ropes flew out of midair like thick snakes, wrapping themselves tightly around the centaur’s torso and trapping his arms: he gave a cry of rage and reared on to his hind legs, attempting to free himself, while the other centaurs charged.”
_{^{- Harry Potter and the Order of the Phoenix, Chapter 33 (Fight and Flight)}}

#Eddie Redmayne says a magical lasso Dumbledore won’t think of.

Eddie: I think that what I love about Newt is that he’s not the greatest wizard in the world, but his skill set is so specific. I mean, Tina and Newt call Grindelwald at the end of the last film using a distraction technique – had to see this one – and the sort of lasso [pronounced “lay-sew”] that Dumbledore wouldn’t have thought to use.

 

Katherine: That’s lasso [pronounced “lah-so”] in our country.

 

[Everyone laughs]

 

Eddie: Magical lasso. [laughs]
_{^{- Eddie Redmayne and Katherine Waterston “Crimes of Grindelwald” Roundtable Interview}}

The screenplay says it’s “a crackling rope of supernatural light”.

“In a split-second NEWT pulls the cocoon from his pocket and releases it at GRAVES. The Swooping Evil soars around him, shielding NEWT and the Aurors from GRAVES’S spells, and giving NEWT time to raise his wand.

With a sense that he’s been holding this one back, he slashes it through the air: out flies a crackling rope of supernatural light that wraps itself around GRAVES like a whip. GRAVES tries to hold it off as it tightens but staggers, struggles and falls to his knees, dropping his wand.”
_{^{- Fantastic Beasts and Where to Find Them: The Original Screenplay}}

“Be quiet!’ shouted Hermione, but it was too late: Umbridge pointed her wand at Magorian and screamed, ‘Incarcerous!’

Ropes flew out of midair like thick snakes, wrapping themselves tightly around the centaur’s torso and trapping his arms: he gave a cry of rage and reared on to his hind legs, attempting to free himself, while the other centaurs charged.”
_{^{- Harry Potter and the Order of the Phoenix, Chapter 33 (Fight and Flight)}}

Eddie Redmayne says a magical lasso Dumbledore won’t think of.

Eddie: I think that what I love about Newt is that he’s not the greatest wizard in the world, but his skill set is so specific. I mean, Tina and Newt call Grindelwald at the end of the last film using a distraction technique – had to see this one – and the sort of lasso [pronounced “lay-sew”] that Dumbledore wouldn’t have thought to use.

Katherine: That’s lasso [pronounced “lah-so”] in our country.

[Everyone laughs]

Eddie: Magical lasso. [laughs]
_{^{- Eddie Redmayne and Katherine Waterston “Crimes of Grindelwald” Roundtable Interview}}

#Eddie Redmayne says a magical lasso Dumbledore won’t think of.

When Eddie Redmayne mentioned what Newt used to tie up Grindelwald in the context of Newt’s specific skill set, he calls it a magical lasso, and says Dumbledore wouldn’t have thought to use it.

Eddie: I think that what I love about Newt is that he’s not the greatest wizard in the world, but his skill set is so specific. I mean, Tina and Newt call Grindelwald at the end of the last film using a distraction technique – had to see this one – and the sort of lasso [pronounced “lay-sew”] that Dumbledore wouldn’t have thought to use.

Katherine: That’s lasso [pronounced “lah-so”] in our country.

[Everyone laughs]

Eddie: Magical lasso. [laughs]
_{^{- Eddie Redmayne and Katherine Waterston “Crimes of Grindelwald” Roundtable Interview}}

He doesn’t say what spell it is specifically, but since he considers it part of Newt’s specific skill set that he’d think to use it, and Dumbledore wouldn’t, it may be intended for use on magical creatures.

#The screenplay says it’s “a crackling rope of supernatural light”.

In the screenplay of the movie that was released along with it, the effect of the spell Newt uses is described as “a crackling rope of supernatural light”. No spell name or incantation is given.

“In a split-second NEWT pulls the cocoon from his pocket and releases it at GRAVES. The Swooping Evil soars around him, shielding NEWT and the Aurors from GRAVES’S spells, and giving NEWT time to raise his wand.

With a sense that he’s been holding this one back, he slashes it through the air: out flies a crackling rope of supernatural light that wraps itself around GRAVES like a whip. GRAVES tries to hold it off as it tightens but staggers, struggles and falls to his knees, dropping his wand.”
_{^{- Fantastic Beasts and Where to Find Them: The Original Screenplay}}

It’s possibly Incarcerous, which conjures rope. However, when its effect is described, this spell doesn’t seem to conjure ropes of light.

“Impedimenta! Incarcerous!’ A few of them stumbled, one or two of them bound in ropes, but those climbing on to the rock behind them merely stepped over or on the fallen bodies.”
_{^{- Harry Potter and the Half-Blood Prince, Chapter 26 (The Cave)}}

#The screenplay says it’s “a crackling rope of supernatural light”.

In the screenplay of the movie that was released along with it, the effect of the spell Newt uses is described as “a crackling rope of supernatural light”. No spell name or incantation is given.

“In a split-second NEWT pulls the cocoon from his pocket and releases it at GRAVES. The Swooping Evil soars around him, shielding NEWT and the Aurors from GRAVES’S spells, and giving NEWT time to raise his wand.

With a sense that he’s been holding this one back, he slashes it through the air: out flies a crackling rope of supernatural light that wraps itself around GRAVES like a whip. GRAVES tries to hold it off as it tightens but staggers, struggles and falls to his knees, dropping his wand.”
_{^{- Fantastic Beasts and Where to Find Them: The Original Screenplay}}

The spell that seems most similar to this one, and that it’s most likely to be if it’s a spell seen already in the series, is the unnamed and nonverbally cast spell that Dumbledore cast against the Dark Lord when they were dueling in the Ministry. It’s described as flame, not light, and it’s unclear what its intended function is, but it seems the closest of the spells we’ve seen.

“This time it was the one-armed centaur, galloping in front of Dumbledore, that took the blast and shattered into a hundred pieces, but before the fragments had even hit the floor, Dumbledore had drawn back his wand and waved it as though brandishing a whip. A long thin flame flew from the tip; it wrapped itself around Voldemort, shield and all.”
_{^{- Harry Potter and the Order of the Phoenix, Chapter 36 (The Only One He Ever Feared)}}

It’s also possibly Incarcerous, which conjures rope. It’s used to capture people and tie them up, like Newt had done with Graves. However, when its effect is described, this spell doesn’t seem to conjure ropes of light.

“Be quiet!’ shouted Hermione, but it was too late: Umbridge pointed her wand at Magorian and screamed, ‘Incarcerous!’

Ropes flew out of midair like thick snakes, wrapping themselves tightly around the centaur’s torso and trapping his arms: he gave a cry of rage and reared on to his hind legs, attempting to free himself, while the other centaurs charged.”
_{^{- Harry Potter and the Order of the Phoenix, Chapter 33 (Fight and Flight)}}

The ropes conjured by “Incarcerous” are regular ropes, not ropes made of light, so it seems unlikely to be this. However, it still seems worth mentioning, since it’s similar in effect, though not appearance.