Mers have long been only sea faring creatures, more at home under the blue waters and trailing seaweed forests then above land in the cloying dryness.
Some Mer have always had a taste for adventure though and so every now and then you hear tales of Mers being on land. Some of them leaving for good, others coming back with strange items and foods.
When questioned about where learned to go on land, they would talk of a strange Mer deep in the trenches who taught them how, talking about rituals and flashing lights before their tails spilt apart into legs.
Young curious Mer would sometimes go visit the trenches, wanting to explore land, before coming back home a few hours later, telling tales of a strange glowing Mer who ushered them out of the trenches saying it wasn't safe for ones so young, and to come back when they were older.
Your heart of home placed by hand
For you wish a single day to stand
Shattered it be upon the sand
Certus is the answer you need from this land
A runic circle with a shattered Heart of the Sea in the middle is all the remains deep in the trenches when a large group of Mer come to investigate