Diancie turns into Mega Diancie when it uses Diancite. It retains Diancie's colors but the lower half is now a dress-like structure with pink crystals extending further underneath. The gem on her forehead has turned a bit more heart-shaped with crystal bangs instead of pigtails. Two long ribbon-like structure extend from the back of her head. All the crystals and the ribbons seem to have tiny rhomboid crystals off the end of them. A larger gold crescent extends around its torso and some extend under her dress-like structure around the giant gem.
Main Pokémon games
Diancie only appears in the games as an event-only Pokémon so, it lacks any notable interactions in the main games