This isn't some stupid theory.
Before Alexandros Mograine died he weild the Ashbringer, when he was killed he was risen as a Death Knight by Kel'Thuzad.
On the Homepage of mmo-champion.com
I saw that pic. Its Alexandros Mograine wearing DK t10. Where do we get t10? ICC. Now, you might think thats Darion because of the Slayer ID blade. Its not. Darion has BLONDE hair. Alex has red. Same with his other son Renault.
I bet money its him because, it makes perfect sense for Arthas to give Darion an obstacle because he wants Darion back with the scourge.
Now, another thing.
During the flashback with Darion and his father. Right before it becomes corrupted, Arthas/Lich King, yells "He is mine now" and a very miniature version of Alex is formed on the floor all dead looking. If you've done the Hallowed event you can back up this theory. If you want
wowwiki Alexandros Mograine.