The “fatal shore” is a metaphor for how the lover will be killed by the lady’s father if he returns to the shore.
This poem is about a tragic love story. In ballad form, we learn how the lady and her lover eloped, but the lady’s father was not happy about it. The lady decides to flee, but a storm is coming. She has to decide whether she will risk her life and her lover’s life on the boat, or whether she should return to shore to face her father’s wrath. She knows that if she does, Lord Ullin will kill him.
Even though her father is the chief of Ulva’s Isle, he is not an important chieftain like the lady’s father. The lover explains that he will be killed if he returns to the shore.
``And fast before her father's men
Three days we've fled together,
For should he find us in the glen,
My blood would stain the heather.
The two lovers are in an impossible situation. If they go on the boat, they risk all three of their lives (the lady, the lover, and the boatman). Yet if they return to shore, the lover will face certain death at the hands of the angry father. In the end, the father is grieved when he watches his daughter drown along with her lover.
The fatal shore is a metaphor for death. Metaphor is a comparison of two unlike things. We describe the shore as fatal, even though it is not literally the shore that is fatal. It is the father, who will kill the lover when they return to shore. Either way, it’s the same for the young lovers. They cannot return to shore