While working on my Tyrian Grandmaster Cartographer, I found a "Forgotten Guardian".
He apparently is a portal that joins Nightfall, and Prophecies. You appear in the Crystal Overlook in Nightfall, to the far west of Ruins of Morah. You must have mastered the Junundu I believe, as he mentions it.
Map:
Forgotten Guardian?
Enchanted Warrior
Mournblade
Yep, old news i'm afraid. People were discussing that about two days after Nightfall was released. 
Malak Revan
Old news like he said but that Guardian is involved in a quest that you get in NF, his partner in NF sends you to Proph so you can finish the quest and this guardian is the way back after you have completed it. The quest is called " Family Soul"
Legandary Spartan
ouch, that was stab at my pride. I thought i found something cool that know one else knew. Is there on to cantha?
Skyy High
Cantha is far across the sea, while Tyria and Elona are part of the same landmass, so no there's no portal to Cantha.
Soul of the Scythe
You can only get the quest if you have been to the crystal desert AND tamed a Junundu.