The Border Sea is an evil location and place in The Keys to the Kingdom series. It was an ocean previously under the rule of the gluttonous Lady Wednesday. It was the third demense that Arthur gained control of as the Rightful Heir.
After Wednesday transformed into a leviathan and submerged into the sea, the sea overflowed nine-tenths of all land in the demesne including Port Wednesday. A new port was partially prepared on Wednesday's Lookout before this deluge. Any of the buildings that were submerged were transformed into ships, which thereafter sailed the Border Sea along with Wednesday’s original fleet of 49 ships. A unique aspect of the Border Sea is that anything that is lost accidentally, but not stolen, will appear in it. Then, this ocean is in a nigh-anarchy state, yet the rule of Lady Wednesday and her remaining minions is still powerful and influential in this area.
The Border Sea extends into any body of water in the Secondary Realms, and therefore can be a means of travelling anywhere. The Line of Storms, a series of vicious lightning storms, guard the entrance into the sea; only an express invitation into the Sea will prevent the Line from killing people instantly.
Raised Rats live here and pilot their steamships. Pirates also live here, of whom the most formidable, twisted and horrible was a mortal pirate who somehow passed the Line of Storms into the Border Sea: Captain Elishar Feverfew. Feverfew was a mortal pirate who entered the House and mastered the art of House Sorcery. He learned many dark sides of sorcery that others such as Dr. Scamandros would not willingly use. Due to his experimentations with Nothing he is no longer fully human, but a Denizen with serious scars left by dark sorcery.
At the end of Drowned Wednesday, Wednesday’s Dawn informed Arthur that much of the Sea had extended into the Void of Nothing, and since Wednesday used all of the Key’s power keeping her shape, had not reined it in. As Arthur does not wish to use the Keys unless absolutely necessary, he left this task to his Steward Dame Primus.
In Lord Sunday, the Sea is revealed to be destroyed by a wave of Nothing. However, it is now very likely remade by the New Architect.