only when he, crowned in cockle shells, takes up his arm

[kttlt]

The relevant timestamp is [0:30].

Bellow, bid our father, the Sea King, rise from the depths, full-foul in his fury, black waves teeming with salt-foam, to smother this young mouth with pungent slime... To choke you, engorging your organs till you turn blue and bloated with bilge and brine and can scream no more, only when he, crowned in cockle shells, with slithering tentacled tales and steaming beard, takes up his fell, be-finned arm -- his coral-tined trident screeches banshee-like in the tempest -- and plunges right through your gullet, bursting you, a bulging bladder no more, but a blasted bloody film now [...]

Source: The Lighthouse, 2019. Directed by Robert Eggers.

What does "only when he ... takes up his fell, be-finned arm" mean here? Should I understand it as "till you can scream no more, except for when he takes up his arm and plunges right through your gullet"?

 

[teechar]

That video is not available any longer.

 

[kttlt]

That video is not available any longer.

On my end it just says "Playback on other websites has been disabled by the video owner", but there's an underlined "Watch on YouTube" hyperlink below. Is that not the case for you?

In any case, I've updated the OP with a transcript.