Fost is a Death Knight. He originates from Aravorn, not from any other realm. In his life, he had an unclean death. He refused to pass into death and so the Under-Realm's dark magic took him. He was summoned back to Aravorn by Jul and Grob.
Fost's spirit currently resides in a possessed suit of armor.
Fost doesn't look like the typical Death Knight. He looks like nothing more than a possessed suit of indestructible armor.
Fost was initially defeated during the events of The Old War. After his defeat, Fost was entombed, sealed away.
It was assumed he would be unable to rise again, but his return heralds the events of Loren the Amazon Princess.
It is revealed the only way Fost could be restored is if every piece of his armor was brought together, and his spirit summoned once more.