The 8th Day was a storyline in which 8 avatars of powerful beings were awakened to battle each other and the winner gets the earth.... Juggernaut was one of the avatars (of Cytorrak) but he had accidentally been awakened years before (by awakened i mean someone had found the item which accessed the power... in his case the gem or "crystal")
7 other people felt this strange pull towards something, each of them found an item and each of them became very powerful in different ways..... but were controlled by their respective entities.... as Juggernauts powers were accessed earlier he hadn't received the full dose of Cytorrak power... when the 8th day began, he gained the rest of the power that was for Cytorraks avatar but Cytorrak attempted to control him like the others were being controlled... he fought back and won.... eventually, with the help of Iron Man, Thor, Xavier and Spider-Man... they stopped the other 7's plans to make the earth a war-zone (by each controlling an 8th of the population)
the other avatars realised they could break free of the control they were under
not entirely sure why Juggernaut has been depowered again though
The power down is linked to the Eighth Day. As Juggernuaght did not suceed in implementin Cytorak's plans, he has greatly diminished his powers. He's still class 100, but he has gone from being about as strong as Namor to as strong as colossus. This is a bid decrease. !!!!!!!!!! His durability is also knowhwere near what it used to be. For example Black Tom was nearly successful at killing him with his secondary mutation.
marvel just wanted to revamp all thier characters all at once in a uniform fashion without having things look outta place or having to explain each charcter individually. for example since the start of sec mutation in early 2000, it is accepted across the entire marvel spectrum that Emma frost can turn into diamond...even in the ultimate universe, and that iceman controls moisture down to a molecular level.
you see i believe the goal is to make it so that in the next 5 or ten years of comics, as new readers are attracted they wont even have heard of a thing called a secondary mutation, to them it would be like thats how the charcters were all along
an old comic where it was just a big description of juggernaut. It came out right around the time of Juggernaut: the Eighth day. It wasnt really a comic though, more like a "Handbook of Juggernaut". The title was just called Juggernaut, so it might be hard to find. The part about current Juggs is a guess though.
It's hard to say no one really knows his real power level. I mean look at what he has done. His only real feats are that he powers Juggernaut and is invoked bt Dr. Strange and that Strange uses items from him. Other than that his actual power level is unknown for all we know he is below skyfather or could be around Galactus possibly.
I think you are getting me mixed up with someone else. Namor has knocked the Hulk out cold, something Juggernaught has never done. He broke the crismson bans of Cytorak, which are harnessed by the same power as Juggernaught. I think he might be.