Spoiler warning: If you don't know what a "mournblade" is, you may want to stop reading and play more of the game first.

You have been warned.

After turning in 15 memories to the Runecarver, you get a cinematic where it is all but confirmed that he created Frostmourne and the Helm of Domination while subjugated by the Jailer, and Frostmourne is one of the "mournblades".

What does a mournblade do? Uses death magic to siphon souls to empower itself. Remornia, Denathrius's blade is probably also a mournblade, and she siphons her owner's soul inside herself once he is killed in the raid.

What we don't know yet is how mournblades are created. What if the Jailer had a way to turn Sargeras's sword into a mournblade that will then be strong enough to literally siphon the soul of Azeroth? He would then possess Frostmourne on crack and be the de facto god of the cosmos with a sword that can cleave planets and kill titan-level beings to empower itself.

And if this is the Jailer's goal, then Ven'ari's obsession with our Heart of Azeroth seems even more suspicious...