Normally, they could only stay on land for a full year. If they failed to return to the depths of the Heartless Sea within that year, they would burn up like spent wood and be reduced to ashes.
Once the Old Ancestor succeeded in soul-seizure and devoured some members of the Blood Demon Clan to strengthen himself, perhaps he could rely on his formidable power to find the formation flags sealing them away, thereby completely freeing the Blood Demon Clan from the destiny of being manipulated by others.
Of course, they would still have to return to the ocean floor after a year. But even if they returned, with the seals gone, couldn't they devour the living beings on land whenever they pleased?
Chapter 349