mystic egg +leadership level up - what does it do?

[DGDobrev]

That is the idea. It gives the egg a fixed amount of leadership + % of your own, so with a high enough level, you can spawn a significant number of dragons to aid you, pretty close to your max leadership value.

Just like Ball of Lightning, this ability must be maxed (or closed to maxed) in order to make it shine. In the beginning, it can provide some scapegoat units, but they're usually not that much.