The poetry of Keats is often mysterious and suggestive of terror.
The description of the Gothic hall in _The Eve of St. Agnes_:

"In all the house was heard no human sound;
A chain-drooped lamp was flickering by each door;
The arras, rich with horseman, hawk and hound,
Fluttered in the besieging wind's uproar;
And the long carpets rose along the gusty floor;"

the serpent-maiden, Lamia, who

"Seemed at once some penanced lady elf,
Some demon's mistress, or the demon's self;"

the grim story in _Isabella_ of Lorenzo's ghost, who

"Moaned a ghostly undersong
Like hoarse night-gusts sepulchral briers along."

all lead us over the borderland. In a rejected stanza of the _Ode
on Melancholy_, he abandons the horrible:

"Though you should build a bark of dead men's bones
And rear a phantom gibbet for a mast,
Stitch shrouds together for a sail, with groans
To fill it out, blood-stained and aghast;
Although your rudder be a dragon's tail
Long severed, yet still hard with agony,
Your cordage, large uprootings from the skull
Of bald Medusa, certes you would fail