From the brief description you've given, it sounds like a massive data dump and I think it's going to be a huge stopping point which might well provide backstory but will absolutely halt the action in the major arc and may well pre-empt your climax.

Can you get around it in another way? Maybe one of the other characters can comment that he has gone to that world for training (or even just that he is absent) and then you can pick up the story again when he comes back and gradually reveal what he's learned along the rest of the story.

If you really don't want to waste your world-building (and I know that feeling, believe me) you could make up a reference document and post it separately so that readers who are interested in that level of detail can read it but it won't break the main story.