Complex sentence: When Zane was still very young, Juggernaut went on a mission in another dimension, the Crossroads of Infinity, with the X-Men and was forced to stay behind to prevent the demon hordes of the Overlords Eight from following Wolverine and the other X-Men back to Earth.
Split into simpler sentences: When Zane was still very young, Juggernaut was on a mission in another dimension with the X-Men. He stayed behind in the Crossroads of Infinity to prevent the demon hordes of the Overlords Eight from following Wolverine and the other X-Men back to Earth.