Sheath of Light was likely found last because it was bugged for a lot of players.Even once its method of acquisition became public knowledge, many of us were not given an option to progress the sequence by Harold Riggs until a hotfix came in an hour or two after potential issues were identified. The fact that the Scarlet corpse was found on Newman's Landing well in advance of the full unlock process being discovered was testament to this.
we need at least 1 new rune per spec in phase 6
phase 1 - buggedphase 2 - all found in less than 48 hoursphase 3 - no one went for it because it was trashphase 4 - bugged follow up questbugs and undercooked content, SoD in a nutshell
bladestorm