Through out shadowland campaign, story should be revolving around a captive soul in maw which jailer using to gain power and dominance over death, however the souls resisting to give all in, Jailer use anduin to subdue him and gain total power, during final battle that souls last remaining essence is all stand b/w jailer plan and world of leaving, player effort and long quest chain across patches finally free him at last moment when all was lost, that souls get free from jailer control along with Anduin and now stand b/w jailer and his plan, while players during final battle almost lost, the soul gain his true form and then fight with jailer with Anduin and defeated the jailer, and at that time its reveal that he is Arthas forever tormented and in-prison by jailer the day he put hands on frostmourne.
All Arthas wanted to save his land, even when consumed by FrostMourne he kept undead from destroying it, and after dead he kept shadow-land destroying leaving world
O Thanagor
An Karanir Thanagor,
Mor Ok Angalor.
Mor Ok Gorum
Palahm Raval.
Ro-mun A'l Ga
Ballog A'l Enthu
Korok Na Boda...