perhaps revan has found a way too die and become one with the force,then be reborn trough the force.
sounds pretty damn strong ,but who wouldnt agree that revan is one of the strongest (grey)jedi ,and also was one of the at that point strongest sith lords aprox. 300+ years ago.
the emperor saw a rare potency in him ,given the opportunity,that revan would rise in power.
The mandolorian wars were that opportunity,and the qeust too find the star forge with the maps further evolved his strength.
even though he was being manipulated and used by the emperor ,a few era in captivity and mind-****-torture didnt break him,he could resist the emperors voice wich in my opinion makes him maybe not as strong as the emperor once was ,but now atleast as strong or stronger then the emperor.