Oke. First of all, we thought Ner'zhul created Frostmourne. Tichondirus said that.
In 'Manual of Monsters' we learned it was not forged from Ner'zhul as Lichking, but given to him from demons. Soo.. from the Nathrezim maybe?
The First Lichking was created from the Legion. Was he maybe created to get entrance to the Shadowlands somehow? So the Lichking is a real danger to the shadowlands.
Could the Nathrezim maybe, be related to the Venthyr? Necromantic Vampires. Could a Venthyr/Nathrezim maybe get the sword and helmet from the Jailer and be ordered to bring it to the Lich King to get control over him, because he didn't want the Legion to get into shadowlands? I mean, there was never a 'undead problem' before the Lichking was created and now suddenly there always needs to be a Lichking?
So finally Arthas dies and the Jailer is waiting for the Lichking, but all he gets is this Arthas guy who was just a Vessel, dumped into the maw like a piece of s*. The real Lichking who is the dangerous part is still out there, hiding in someone else body and suddenly Sylvanas pops up, because she jumped from the Frozen Throne and the Jailer is like "Bring me the real Lichking or eternal pain awaits you! The Lichking is sticking to someone alive out there" So Sylvanas starts her thing and sends as many souls as possible to the Jailer in hope to find the one corrupted soul and in an act of desperation she finally decides to send everyone there.
Take this with a grain of salt, I just think we are at some point were everything is possible.
But could anything of this be true or be part of the whole big unknown picture?